IT'S THE end of the autumn term at NUI Galway. In the Galway Bay Hotel, the lobby area is swarming with students - chattering, bickering or doing lastminute revision - for (a blackboard by the wall tells us) NUI Galway has chosen this plush venue for its scholars' end-of-term exams.

Through the milling throng, looking wildly out of place, strides a familiar figure. He is short and strikingly handsome, clad in black shirt, black jacket and black trousers; his face is oddly unlined, the vestiges of an Elvis quiff dance on his forehead, and he looks a lot younger than his 66 years.

Strange to report, although he's a world-famous actor instantly recognisable to film and TV viewers of two generations, his presence elicits no squeals of celebappreciation. As Martin Sheen pops outside for another of his hourly fag-breaks, you'd think he was one of the students. And you'd be right.

The folk of Galway collectively rubbed their eyes when they read in the Connacht Tribune in August that the legendary star of Apocalypse Now and The West Wing was coming to study in their midst. The man who slaughtered Marlon Brando's deranged Colonel Kurtz and who, as President Josiah Bartlet, weekly solved problems of global diplomacy and domestic mayhem - he was coming here, to be a student and hang out in local bars? Jaysus. Would he go native? Would he spend his time having essay crises, living on pints of lager, writing poetry to cruelly beautiful girl students slightly out of his league and going to folk gigs?

Sheen laughs at the idea. "No, I'm living the life of the over-privileged. I took an apartment in Salthill, just up the road. But I'm allowed to roam free. I have a mentor, a young man a third of my age called Luke Devlin, he looks after me and makes sure I get the notes if I miss any lectures." He chuckles, as he regularly does during our talk. "It's been one of the great adventures of my life. My only regrets are that I didn't do it earlier and that I can't do it any further. I just love this country and the experiences I've had here."

He's become accustomed to the people who approach him with the words "Is it yerself?"

"I met this kid who said, 'Where's yer minder?' I asked him, 'What's a minder?' and he said, 'You know - yer bodyguard. Yer t'ug [thug].' I love that, t'ug!"

But what's he doing here? He has been coming to Ireland since 1973, when he first met his cousins from his mother's town of Borrisokane in Tipperary. But the impulse to study came from elsewhere. "Fr Joseph Power, my old friend and pastor from the Church of Our Lady of Malibu [in California], retired and now lives in Athenry and I wrote to him two years ago expressing a romantic fantasy about coming over to Ireland to study. He said, 'Come over and we'll check it out.' I did and, under cover of darkness, we looked at the campus and the curriculum.

"I decided that I would come whenever we got a window in The West Wing. Coincidentally, the next spring I was offered an honorary degree by Galway. I came over and the president asked, 'What are your plans?' I told him 'I have enough degrees. What I really want is an education.'" And that was that. He was admitted for one term to read English, philosophy and oceanography. Keen to study environmental issues, he discovered he didn't qualify as a science student, having no qualifications in biology, or calculus; but they let him attend lectures nonetheless - "and every class was like another instalment of An Inconvenient Truth." He was delighted to pass the beginner's certificate in computer studies ("My grandchildren think I'm from the Stone Age") and he was worrying about his final English essay, on The Merchant of Venice ("I knew nothing about Shakespeare, even though I've done a few of his plays on stage"). For philosophy he read "Socrates and Plato, all the way to Nietzsche by way of William James and pragmatism. I didn't have any understanding of the roots of the subject. In the bookshop, I bought so many books they asked me if I really intended to read them all."

As his stint in Studentland draws to an end, Sheen can reflect that he's had quite a year. Last May, The West Wing closed its doors after seven years of quality drama set in the murky corridors of power at Pennsylvania Avenue, and the career of Josiah Bartlet - America's favourite "acting president" as Sheen often referred to himself - ended too. A year ago, he was filming Bobby, an ambitious movie about Bobby Kennedy's assassination, directed by his son Emilio Estevez, which opens just in time for the Oscars season.

Sheen also found time to appear as the good police chief Oliver Queenan, in The Departed.

Now he's a born-again college boy, who - at the time of our meeting - is about to go home for Christmas with his wife Janet, to whom he's been married for over 40 years.

Things have worked out rather well for the one-time Ramón Estévez, one of 10 children born to a Spanish immigrant factory worker and his Irish wife in Dayton, Ohio. A life-long rebel who left school at 18 and grabbed a bus to New York with a ticket paid for by the local Catholic priest, Sheen is now every inch the sleek paterfamilias, helping out the children in their directorial careers.

Bobby is a multi-narrative patchwork of a movie, interweaving a dozen story threads to evoke the lives of people living and working in the Ambassador Hotel, LA, on the day - 4 June 1968 - Kennedy was shot in the lobby.

Their individual stories deal in various traumatic issues - racism, infidelity, drugs, Vietnam - and offer roles that are essentially soap-opera parts to a score of Hollywood heavyweights, including Anthony Hopkins, Harry Belafonte, Heather Graham, Sharon Stone, Demi Moore, William H Macy, Elijah Wood and Laurence Fishburne.

Sheen's role is slightly puzzling. According to the production notes, he plays a depressed east-coast millionaire "examining the very roots of his modern malaise" while his wife [Helen Hunt] goes shopping. In fact, Sheen plays a slightly subdued version of himself, but cannot help allowing cheerfulness to keep showing through - possibly the pleasure of being directed by his own son.

"It's not the first time, " he says, beaming with pride. "I played his father when he directed The War at Home a few years ago in Texas. It didn't get a big public play, but it's revered in the industry. We'd acted together in a couple of things previously and I learnt to be ruled by him completely. He has a wonderful instinct with actors. I watched him one morning directing Kathy Bates and thought, 'My God, this is one of my favourite actresses in the world and she's being ruled by him'."

Originally, Sheen had been slated to play a larger role in Bobby. "But as time went on Emilio realised he couldn't get me out of The West Wing long enough to interact with lots of other actors. The only role where I'd be with only one player was the role of the rich guy. Emilio said, 'It's such a small part'. I said, 'I'll play a waiter, I'll do a walk-on and be as happy as Larry'."

He was in good company. Once Anthony Hopkins had agreed to play a doorman, a seething mass of Hollywood Alisters came on board. "They were aware that powerful work was being done. There were people coming in who I didn't know. People were making a big fuss over this little girl. I said, 'Who is that?' They said, 'Lindsay Lohan, don't you know her?' I said, 'Never heard of her'." Sheen chuckles.

"It's a testament to Emilio's personal vision that people were attracted to it and that, in the words of one critic, 'Estevez took 22 stars and made them nobodies'" - he frowns as if not entirely convinced this is a compliment.

"And it was to his credit that he got them to play ordinary people." It's certainly a credit to Estevez's force of character that he got Sharon Stone and Demi Moore to play two raddled old bags - Stone, in particular, disappears so totally inside her part as the hotel's hairdresser-cum-manicurist, it's hard to recognise the glammy ice queen of Basic Instinct.

Sheen was in the middle of filming, "just as I was rubbing lotion into Helen Hunt's back", when he heard the news that John Spencer, who for seven years played President Bartlet's secretary of state, Leo, had died. He still regrets not being able to join his co-actors at the hospital. There's a catch in his voice when talking about Spencer and the rest of the West Wing cast, as if he's discussing close relatives.

It's hardly surprising when you think of the hothouse atmosphere in which the nine main characters lived and acted for so long. That may be one reason for its success - that we got to know them and all their complex interactions, as well as we know our own friends.

Did Sheen and Bartlet share similar political convictions? "I've often been asked, 'Would I have been able to play Bartlet if he were a Republican conservative?' I always said, 'It depends on the script. If it was an honest script, I'd have no problem at all doing that.' But as it was, he's Democrat liberal, which fitted me, but there were many things I had to do as Bartlet which I wouldn't approve of as Sheen."

Such as the time the president signs a death warrant?

"Yeah, exactly - I'm opposed to capital punishment, I'm on the record as such, and when I got to the episode about the death penalty, when Bartlet is faced with the choice of commuting it or allowing the guy to be executed, I told my attorney - he's a young public defender from Pennsylvania, a national spokesman against the death penalty - 'I can't do it'. He said, 'You have no choice. You'll lose all credibility as Bartlet if you press for a stay of execution'." Sheen's face is full of pain as he wrestles with his true identity and his screen one. "I love the fact Bartlet works from a political as much as from a moral frame of reference. Now me, as Martin Sheen, I would object and wish that I could be heroic and let everyone live and not be the cause of anyone's life ending; but [as an actor] I just didn't have that luxury."

It was back in September 1999 that The West Wing burst upon the viewing public, the brainchild of Aaron Sorkin, who wrote the 1995 movie The American President, and worked some unused ideas from that film to get the new show underway. Seven years, 156 episodes and a recordbreaking 26 Emmy awards later, it's piquant to remember that President Bartlet wasn't originally meant to be at the centre of the action. Rob Lowe, as Sam Seaborn, the deputy communications director, was supposed to be the emotional focus of the plot, with the president remaining a mostly unseen, looming figure elsewhere in the White House. From the outset, though, Sheen invested the president with such capricious, quixotic, sagacious charm, you just knew he was bound to take over the show.

His first line in the very first episode was significant:

entering an office to meet a delegation from the Christian Right who are squabbling about the First Commandment, he intones, "I am the Lord thy God, thou shalt not have false Gods before me, " and they soon crumple before his withering contempt. As written by the brilliant Sorkin, Bartlet/Sheen became a weekly political vaudeville turn with frequent excursions into sainthood. When the president, in the middle of a state dinner, talked on the telephone to a nobody American sailor on a stricken ship in a hurricane, you could hear the viewing nation hold its breath and wish they had a head of state like Bartlet.

In 1999, they nearly did; they had Bill Clinton. A year later, they had George W Bush. "I adored Mr Clinton, " says Sheen warmly. "And he was a fan of The West Wing. For a while we were given carte blanche at the White House, which ended when Bush got in - ended for me, anyway.

When the new administration got into power, all my West Wing colleagues were invited there to meet their counterparts. All except me. I was specifically asked not to come. I was very relieved about that."

Did Sheen think Bush ever watched the show, just to see what pinko liberal drama looked and sounded like? "I don't know. But I wouldn't call it pinko liberal drama. I'd call it wishful thinking at worst, and good - I want to call it good fantasy - at best." He is predictably scathing about Bush, as much for his mismanagement of small domestic events as for global catastrophes. "He has this image, that he's a regular guy you could sit down and have a beer with. But it's all an image. Take the Dixie Chicks - you know, those girls who said, around the time of the invasion of Iraq, 'We're ashamed to be from the same state as Bush'? Now if you're the president, and you're sitting on a 70% approval rating, you just let these girls hang out to dry. They're getting death threats, their CDs are being burned and concerts cancelled. And you say nothing, because that's who you are. Or if you're clever, you say, 'This is an opportunity, let's invite them to the next barbecue and if they don't turn up we'll just play their records, because by God I love their music and I don't care how they feel.' But he couldn't leave it alone. He didn't have that kind of heart."

Did you see what happened just there? Sheen turned into Bartlet and compared his strategies and tactics with those of the real-life president. He has, of course, been asked a million times if he fancies his chances in politics, if he'd like to follow his fellow thespian Ronald Reagan and launch an assault on the presidency. Sheen always says no but is quick to embark on tirades of political denunciation against matters of social or racial injustice. He has been a radical firebrand for years, the first man at the barricades. He is immensely proud of having been arrested on 70 occasions.

"The happiest day of my life was when I was arrested for non-violent action against Star Wars with Daniel Berrigan in New York in 1986." Berrigan was a Jesuit priest in New York who opposed the Vietnam war and went to prison for his beliefs. The happiest day of your life? Really? Why?

"Because I'd done everything I possibly could to object to this monstrous programme. I'd done it in public, nonviolently, and I was willing to accept the consequences. It was the most freeing experience imaginable."

A chronic fighter for truth, justice and the American way, Sheen has long held a candle for the Kennedy clan. He played Bobby in a film, The Missiles of October, in 1974. He played President Kennedy in a mini-series in 1983, and his voice provided the narration behind Oliver Stone's JFK.

And yes, he did meet Bobby; in fact he introduced him to Bobby's director. "Emilio was just a couple of years old, sitting on my shoulder, when he shook hands with Bobby. It was 1964, during the campaign for the Senate. I was acting on Broadway, Bobby was scheduled to attend a rally in the old Madison Square Garden and I was asked if I'd join it.

Selfishly, I said, 'I'll come if I can meet Bobby Kennedy'.

And I did. I was on the dais with him for three hours."

Sheen in oracular mode is quite something. You learn not to get him started on politics, or you'll be given 20 minutes on weapons of mass destruction, or on matters of religion (25 minutes on the mystery of the Eucharist). But his willingness to talk a blue streak on matters of belief and conviction is part of an admirable project of selfbetterment that you don't get with many actors, or many sexagenarian millionaires.

After playing a hyper-intellectual, preternaturally wise political titan for seven years, Martin Sheen has learnt the value of real thought, real work, real struggle. "You want to know where it all comes from? Both my parents were immigrants. I became a pro golf caddy at nine years old, at this very exclusive country club for the overprivileged. So I learnt what not to be, by watching them. I learnt the true value of honest labour. I learnt very quickly that the lost causes are the only ones worth fighting for. And, as a result, I'm never comfortable unless I'm uncomfortable. Or in prison."

The three acts of Sheen 1940 to 1979

Martin Sheen - born Ramón Gerard Antonio Estévez on 3 August, 1940 - grew up in Dayton, Ohio, the seventh of 10 children. Having deliberately flunked his college entrance exam so he could pursue a career in acting, he left for New York in 1959. Once there, Estévez adopted the stage name of Martin Sheen. After a few years of auditions and odd-jobbing - during which time he married his wife of 45 years, Janet - Sheen landed his first major Broadway role, before making his feature debut in 'The Incident', in 1967. Three years later he moved with his wife and four young children to Hollywood. His next movie milestone, in 1973, was Terrence Malick's acclaimed 'Badlands'. It brought him to the attention of Francis Ford Coppola who offered him a starring role in 'Apocalypse Now', as the troubled Captain Willard. It was a film that propelled Sheen into the stratosphere.

The genre-defining Vietnam movie was as epic as the tempestuous tales behind the scenes, with a rampantly alcoholic Sheen barely surviving the 18 months of filming in the Philippines jungle. Six months in, Sheen suffered a near-fatal heart attack. He was just 37. The film, released in 1979, won two Oscars and the Palme d'Or.

1979 to 1999 With the booze taking over, 'Apocalypse Now' didn't herald the golden age many had predicted for Sheen. His private life descended further into turmoil as he briefly walked out on his wife and children, and continued his excessive gambling and drinking. The slow road to recovery began in 1981, while he was shooting 'Gandhi' (Sheen plays a reporter) on location in India. Here, after meeting Mother Teresa, he rediscovered his Catholic faith. Sheen subsequently decided to give up the booze, joining Alcoholics Anonymous in the mid-1980s. After 'Gandhi', Sheen made the decision to devote more time to his family and to his developing commitment towards social and political causes. In his next major movie, 'Wall Street' (1987), he starred opposite his son Charlie Sheen, before featuring in the Irish comedy, 'Do', the following year. Towards the end of the decade, as Sheen was coming to grips with his own demons, Charlie appeared to emulate his father's wild ways, becoming addicted to cocaine and heroin, which led him to suffer a stroke. Following a slew of forgettable roles in the 1990s - critics seemed split over his performance as General Lee in the 1994 civil-war epic 'Gettysburg' - Sheen put in a solid, supporting-role performance in 1995's 'American President', written by Aaron Sorkin - the man who would later create 'The West Wing'.

1999 - now In 1999, Sheen landed what was intended to be a peripheral role in the 'The West Wing', as President Bartlet. Sheen made it his own and became the show's star. The series went on for seven years, scooping 26 Emmys and garnering thousands of column inches in critical praise. During its run, Sheen also had time to appear in 'Catch Me If You Can' and 'The Departed'.

As well as establishing Sheen as an acting heavyweight, the role also furnished the firebrand political activist with an aura of political authority, energising his real-life campaigns. And while Sheen has apparently commented only a fool would think a TV actor might make a good president, this hasn't convinced the Democrats. Last year, 'The New York Times' reported that party members in Ohio tried, unsuccessfully, to persuade Sheen to run for senator.

For Sheen and country Animal rights Sheen's association with Peta (People for the Ethical Treatment of Animals) goes back a decade.

He has been involved with campaigns ranging from helping the organisation to launch turkey substitute (though Sheen isn't a vegetarian), to joining a campaign to free six polar bears from a cramped cage in Puerto Rico.

Most recently, he successfully helped spearhead a drive to ban foie gras in California.

Seal clubbing The prolific grass-roots activist has been involved in a number of other animal-rights causes, including, in 1995, heading to the remote Magdalen Islands off the east coast of Canada to protest against sealing. Accompanied by veteran campaigner Paul Watson, the two were attacked by an angry mob of sealers who surrounded their hotel.

US militarism The majority of Sheen's 70 or so arrests over the past 20 years have occurred while campaigning against US militarism. His first arrest, in 1986, was at a nuclear test site in Nevada, but Sheen has also been arrested outside the Pentagon (in 1995) and at Vandenberg Air Force base (2000). His stance over Iraq has been particularly vocal.

Fair pay for migrant workers Sheen has also been a vocal champion for the rights of migrant mushroom pickers. As the sponsor of a campaign by the Migrant Rights Centre, Sheen has called for better treatment of migrant workers, who - on about ¤2.25 an hour - earn well below the minimum wage.

'Bobby' is now on general release