IF you still think of Queer as Folk when you hear the name Aidan Gillen, it's time to change the channel. True, his star turn in the Channel 4 drama as Stuart Alan Jones, the promiscuous gay advertising executive, brought him fame and acclaim but eight years on it's his role in an American crime series that's making critics and viewers sit up.

The Wire is HBO's hard-hitting, complex depiction on law and disorder in Baltimore, in which Gillen plays Councilman Tommy Carcetti, the ruthless ItalianAmerican politician who wants to be mayor. The creation of Emmy awardwinning TV writer Ed Burns and former journalist David Simon, its panel of writers include some of America's finest scribes, such as Richard Price, who wrote Clockers, and Dennis Lehane, who wrote Mystic River.

It can at times make for uncomfortable viewing and is more like a novel than any TV programme you've ever seen. Time and Entertainment Weekly are just two of the publications who cited it as the best show on television, although the audience figures haven't matched the critical acclaim. In late March, Gillen will return to Baltimore to start filming the fifth series of The Wire but not before Irish audiences have an opportunity to catch him at the Gate Theatre, as 'Teach' in the David Mamet play American Buffalo.

Domhnall Gleeson and Sean McGinley also star in this dark and witty piece about three small-time hustlers planning a heist of rare coins. It's a role Gillen is obviously relishing.

"Teach is unsettled, unsatisfied, has been fucked over many times, I would say, and things have not gone his way but he's got this optimism and drive, " is his interpretation of his character, who he thinks is appealing even if his actions aren't always admirable. "I like him. I have to like whatever character I'm playing, whether I'm killing people or worse."

It's a revealing statement into how immersed he becomes in his work, considering the number of morally skewed loners that he's played. On occasion it's given him pause for thought. There was a series in Canada that he did, playing a serial killer who manipulates other people to commit his murders.

"We were doing a scene where really young girls were playing the daughters in a family. One by one their father was calling them upstairs and killing them.

It suddenly hit me that this is really insane, what I'm doing right here, because I totally believe it. I hadn't questioned, I didn't really have a moral standpoint on it."

But meeting the actor in person, it's not difficult to imagine the intensity and fervency he brings to his acting. Dressed in a battered leather jacket, looking younger than his 38 years, he thinks carefully and takes a long time before answering each question, punctuating his answer by dishevelling his hair. How does he approach a role? Long pause.

"Depends on what it is. It's all in there, the script. It's different every time. I can't say I've got a particular process. I probably do, I just never thought about it. I don't think about it too much and that's a kind of method in itself. I'm not a great researcher really. You can usually find everything you need in the script - different strokes for different folks. People do it different ways. I'm just lazy." Then he corrects himself. "No, in some ways I could be, in other ways I'm not. I couldn't think of anything else [American Buffalo] for the last two months so I'm not lazy." A little obsessive maybe? He could be - a bit - he concedes.

Whatever Gillen is, his choices as an actor haven't been conventional. While the success of Queer As Folk opened up avenues of new possibilities for him, he chose the parts that interested him and not necessarily the ones that would have meant greater exposure or bigger money. "Some of the things I'm most proud of, they're barely seen because they're a bit kind of off-kilter and just not mainstream, but I don't regret doing them, " he says. "I did the one I wanted to do and I stand by the decision. I've really tried to be careful to take the best thing out of what's on offer."

Gillen has never known a life outside of acting. Brought up in Drumcondra, he started acting in plays in the Project Arts Centre in his teens. Some small film roles were followed by performances in the Billy Roche play A Handful of Stars and other stage work, but a move to London proved to be a clever career move, resulting in a steady stream of work in theatre and film. Looking back, he thinks he possibly should have broadened his horizons.

"The first couple of years I was living in London, I probably should have done something else as well, just to give myself some kind of a social life. Anything. . . I spent far too much time on my own the first couple of years in London where I could have been living my life with other young people, " he says. Post Queer as Folk, he worked with two Irish greats:

Peter O'Toole in The Final Curtain and Richard Harris in My Kingdom. O'Toole, he says, is an "old-school, old-world debauched gentleman" and typically he has a great Harris anecdote. "I was really glad that I did it and that I got to know him a little bit. I got a good night out of it.

I was summoned to the pub. He said, 'O'Toole said you're alright'. So we went out to this pub in Liverpool, down the docks, " he remembers. "There was a queue of people getting him to sign A�10 notes, which he kept doing and winking at me, not conspiratorially but more like, 'This is where I'm at and nobody is asking you to sign A�10 notes'. He bought every round. Of course we had to do our fucking scene the next day.

I think it was part of the fucking test - can you stay up till five in the morning and do this scene tomorrow at seven. Which worked out fine?" It was his appearance on Broadway in Harold Pinter's play The Caretaker that got him both a Tony nomination and the attention of one The Wire's producers. He's grown to like Baltimore, where he'll spend the next five and a half months filming, although it took a while for him to get a handle on the place - the poverty, how half the city is boarded up with burnt-out houses and the crack vials on the street. "I only really knew New York and Los Angeles. It was another kind of America, " he says.

But he believes there's a real feeling in the city that things are going to get better and he likes the inhabitants: "The people have that kind of gritty character, humour and that underdog thing."

The two months he's spent in Dublin in preparation for American Buffalo is his longest chunk of time here in years. But he hasn't spent so much time in London over the last two years either, which is an issue because his two children, Joe and Berry, are there at school. "It is a sacrifice because you do miss a lot when kids are young. You can go away for three months and come back and something has changed forever, " he says. "But they're okay with it. I don't think it's a barrel of laughs all the time but they understand what I'm doing and I just try and make it work for them as much as I can. That's all you can do."

On the career front, however, there appear to be no grey areas or doubts.

Looking back, is there anything he would have done differently? He pauses for a while before answering. "Sometimes you regret a choice you might have made and if a thing doesn't work, I can generally see why and I'm keen to get onto something else. I think I've been pretty lucky. There haven't been too many fuck-ups."

'American Soldier' runs at The Gate until 10 March; the fourth series of 'The Wire' starts on TG4 on 19 March at 11.25pm