Now, the word is that Farrell is putting much of his energy into staying sober, since he went into rehab in December, suffering from what was euphemistically described by his publicist as "exhaustion".

The budgets for movies such as those which Farrell has been involved with can be anywhere up to $30m. If a day's shooting is lost because the lead actor fails to show up, or shows up late or hungover, it still costs hundreds of thousands to pay the rest of the crew. These days, Hollywood execs absolutely refuse to take this kind of chance with their talent, no matter how large or fragile the ego, and many of them now hire the equivalent of a "professional buddy" to more of less babysit actors whose tendencies to excess can cost a production dearly.

Whatever about his hellraising image, which puritan America watched like a car-crash . . . impossible to look at, impossible to look away . . . Farrell was generally known for being reliable and professional. "I work my arse off. I'm never late, ask anyone, " he said two years ago. "I feel neither the pressure nor the grandeur of the situation. I'm still trying to find my feet as an actor. I know it ain't brain surgery, but it confuses me and it comes between me and my sleep."

During the shooting of Oliver Stone's Alexander in Thailand two years ago, Farrell faced the ire of the director on a number of occasions. With three crucial days left to film, Farrell fell down the stairs, breaking a wrist and ankle, after a massive party thrown by his co-star Val Kilmer. Stone was incandescent.

He also ran up a hotel bill of $64,000 during filming in Morocco. Nothing too shocking about that, as he has taken the approach of an old-style rocker to his private life. Let's face it, most people find these kinds of stories infinitely more entertaining than tales of being taken in by the Scientologists.

The official word put out by his publicist, Dannica Smith, was that the actor had developed a dependency on pain killers following his injuries in Thailand, which led to his "exhaustion" by the end of last year. It's much more likely that he was warned by the notoriously fickle studio heads that the days when the excesses of people like Richard Harris and Richard Burton were considered the norm were over, and that his own days as an excessively overpaid star might be numbered.

Since leaving the treatment clinic in Los Angeles, Farrell has been living, if not quite a monastic existence, then a dramatically more low-key one. An insurance company, working for the makers of his latest movie Pride and Glory, has hired the AA equivalent of a sponsor to help keep him sober. Described as a "professional buddy", the man, who begged anonymity and refused to be interviewed for this piece, did say he had worked previously with Mickey Rourke and Sylvester Stallone, for precisely the same reasons.

According to my source, who lives next door to Farrell, the actor spends most evenings at the house of his "buddy" watching television, and has allegedly become withdrawn in his new, sober state. He has dropped the myriad hangerson who have peopled his life since he became a star. In the past few weeks, he has gone to New York to film Pride and Glory and his paid "buddy" and wife have gone to stay with him for the duration of the shooting.

It seems he is taking his sobriety deadly seriously. His family are also said to have been very supportive of him at this time and his mother has been spending more time with him in the US. His sister Claudine has worked as his personal assistant since he moved out here in 2000.

The actor is said to be a devoted father to his two-and-ahalf-year-old baby James and it has been rumoured that he has become involved again with the baby's mother, Kim Bordenave. Last November, Access Hollywood reported that he had bought the infant a $90,000 red sports car.

It's pretty understandable how someone as accomplished as Farrell, who is still not even 30, might have his head turned by the strange people who run Hollywood and the lackeys and groupies who cleave to the stars.

"I'm not seduced by it at all, I swear to God, " he claimed in an interview with People magazine. "I'm easily pleased, don't get me wrong. I'll indulge it. I'll be in Los Angeles for two weeks and I'll have a laugh, get battered, have a buzz, but at the end of the day, I'll go home."

But this is not strictly speaking true, as he has been involved with all sorts of public liaisons of questionable taste, including one famous night in New York when he managed to spend $10,000 in a single evening in a lap-dancing club.

He has been linked with a seemingly indiscriminate selection of lovers from Britney Spears to Playboymodel Nicole Narain. Briefly involved with Miss January 2002, he must surely be rueing the day he participated in the videotaping of their sexploits, which she has been trying to sell for $3m dollars since last summer. Quite a big payday for a night in the sack with Colin Farrell, but hardly model behaviour!

But Farrell hasn't taken the model's marketing efforts lying down, so to speak. In an attempt to stop the distribution of the tape, which he said the two had agreed to keep private, he filed a lawsuit against Narain. This week, a judge in Los Angeles ruled in Farrell's favour and directed the model's lawyer to reply to the actor's lawsuit.

She had claimed that as "co-creator" of the tape, she has the right to reproduce, market and distribute it, under copyright law. It would appear she is attempting to emulate Pamela Anderson, the sales of whose sex tape with Tommy Lee generated major income and notoriety for her.

But it's unfortunate that for Farrell, the existence and circulation of this tape just hands the puritans in the US another stick with which to beat him. There is a feeling among some in Hollywood that this is further come-uppance for the 29-year-old hellraiser and that this kind of behaviour is all very well for someone like Paris Hilton, who is famous for being famous, but quite another for an outsider who has been championed by major Hollywood directors and has risen up the ranks at the pace at which he has.

Farrell's lawyers claimed last July that the release and distribution of the tape would cause irreparable damage to his career. While a court order restraining the commercial release of the video has been put in place since then, it is already widely available on the internet. People in the industry have said it makes him look like a joke or a cartoon figure.

"It probably sounds highly hypocritical, and it is, but this kind of tape becoming public will probably help some bimbos to become known and maybe to get some work in the porn industry, but it's really not the sort of behaviour the studios want to see from an A-list star like Colin, " said Robin Coons, an executive producer at Dreamworks. "Colin really should know better." For all the sex and violence Hollywood churns out every year, it can be a very prim and judgmental place.

This perhaps goes some way to explaining why some of those around Farrell are now so protective of him, or at least so silent. In writing this article, many people who loudly claimed to be his close friend during his stellar rise were suddenly tongue-tied now that he's in a spot of bother, and would say nothing at all, even about his struggle with booze and drugs and sincere attempts to get sober.

His publicist displayed more rottweiller tendencies than was strictly necessary in response to some basic enquiries about the actor. Irrespective of whatever questions were put to Dannica Smith this week, she kept repeating the same answer, mantra-like, through audibly gritted teeth. "Colin is not doing any publicity right now. We'll let you know when he is, " she said.

Colin Farrell has complained of suffering from chronic insomnia ever since he was 12 years old. It's hard to say whether he is losing any more sleep over this current embarrassment, but amusingly he is on record as saying his favourite book is Irvin Yalom's When Nietzsche Wept. Nietzsche also suffered from insomnia most of his life and used to promise himself he could commit suicide in the morning to try to lull himself to sleep.

"The end is far from nigh for Colin, " said Hollywood reporter writer Susan Hornik. "He's probably doing the right thing now by lying low and getting on with staying dry. If he stays on the wagon and manages to find religion and goes on Oprah's sofa to talk about it, he'll be fine!"

It will be interesting to see how he gets on with his new sober lifestyle. He is known to love music, and recorded a version of Bobby Fuller's 'I Fought the Law' which was used in the movie Intermission.

His next project is on a movie of the life of Bob Dylan, where he will play Dylan at one stage in his life. (Nine other actors, including two women, will play other stages or phases of the singer's life. ) He and Russell Crowe talked about setting up a band and diverting from acting a few years ago.

The fact that he failed an audition for Boyzone will probably do him no harm at all.

"I do have the ability to explore life and to be over the moon at the smallest things . . . a few pints and the craic in the pub and I'm in heaven, " he writes on his website. "But I do have a melancholy side to me as well. Acting allows me to feel things, it kind of buys me human experience."