Inside most English teachers is a frustrated actor waiting to get out. Most are content merely to regale the class with fruity-voiced readings from the GCSE Austen or Shakespeare, but not Brendan Gleeson (right). At the ripe old age of 34, the Irish teacher packed in his job at a Dublin secondary school and decided to become a full-time thespian.

Since his dramatic decision in 1990, Gleeson has enjoyed the kind of success most other actors don't find in a lifetime. Moving effortlessly between stage, TV, and films such as Michael Collins, The Snapper and Braveheart, Gleeson has become one of Ireland's foremost faces. Indeed, so prolific has he become that one journalist has dubbed him "the Irish Depardieu".

As Gleeson ranges into a room in the Dorchester hotel, it's easy to see why. Tall, broad and well-built, he cuts an impressive figure - his round face filled with good humour. Having just dispatched a plate of bacon and eggs, he's here to publicise his latest movie, The General, in which he plays the eponymous Dublin gangster, Martin Cahill. A huge hit at Cannes, the film won Best Director for John Boorman, while Gleeson narrowly lost out to John Mullan in the Best Actor category. First, however, Gleeson explains why it took him so long to swap school for scripts.

"I was very suspicious of the acting world," says Gleeson. "I had a notion that it was nasty and hypocritical. That was completely confounded when I did become an actor. I've never met so many wonderful, genuine people in all my life. But, also, there was the fear that I'd actually warp what I loved by having to make a living from it."

As a result, Gleeson went into teaching, married his wife, Mary, had four children, and spent the next 10 years restricting his acting and directing to the school holidays. "It worked for a while, but in the end teaching demands your full attention," he smiles. "The last six months I clock-watched, and the golden rule is, if you look at the clock, you get out. First of all, kids deserve more than that. Secondly, they'll rip you apart if you've lost that authority. I mean, the savagery of a tribe of kids is unreal."

Authority is certainly not something Gleeson's short of on screen. Despite being told by financiers to find a bigger name for his film (Gabriel Byrne and Gary Oldman were suggested), director John Boorman was determined Gleeson should take the lead, and his persistence has paid off. Gleeson's bravura performance as Cahill brims with charisma, but, Gleeson explains, this was achieved only after much soul-searching.

"I felt trepidation about why we were dealing with this guy. Why make a film about someone who hurt people? I had long discussions with John and we came to the decision that we wanted to humanise Cahill but not glamorise him. I have to say I don't fret about how people will react to the film now, because every question I'm going to be asked I've already asked myself, so I feel centred in myself about why I did it."

After squaring his conscience on the subject, Gleeson began to research his role, talking to those who'd met Cahill over the years, and trying to piece together a picture of this complex character. At times, Gleeson admits, it became something of a "dark journey". Cahill was, after all, a man who thought nothing of performing a DIY crucifixion on a gang member he suspected of disloyalty.

"I had nightmares during my preparation for the role. Obviously it's necessary to go into those scary areas if you're getting into character, but the strange thing was that once we'd started shooting, all that disappeared. I felt like a real human being, like I had the control that he had and the wit that he had, and, you know, it was incredible. I kind of missed Martin when it was over. He became a part of me and I came to like him an awful lot."

Like many gangster films, The General portrays Cahill's rise and fall with an almost tragic fatalism. "I think the tragic element in The General is bound up in the fact that this is a guy who had so much to give," says Gleeson. "The way he went was a tragedy for everybody involved. Ultimately, that's a terribly moralistic thing to say, and, in a way, I think the film is very moral. It doesn't make judgements about Cahill as a person, but it shows what a disaster it is that his kind of talent and wit was wasted."

As a teacher, Gleeson says, he had plenty of opportunities to witness young boys taking the wrong path. "You can see those lads a mile off, but sometimes there's absolutely nothing you can do about it. I remember having a discussion with a guy who was about 16 who was going to leave school. He came from an unhappy home and he'd been in trouble all the way through school. I tried to get him to stay, but he was determined to go. This was an extremely bright guy who was getting into a situation of indolence and you could see it was going to be a disaster for him. I talked, cajoled, everything, but nothing would change his mind. That's when I had to go home and tell myself, 'You're just a teacher. You're important in terms of his life from maybe nine o'clock till four, but his life is so much more than that.' Your influence is just so small. It's awful."

Over the years, many other pupils benefited from their teacher's powerful passion for drama, however. "I was very proud that I managed to get almost everybody involved," says Gleeson. "The year before I went into acting full time, I put on a concert with about 300 lads," he remembers. "Honest to God, I laughed so much. There were these fairly tough guys coming out and singing Christmas carols - their cheeks bright red - and I started laughing and couldn't stop. They were looking at me laughing and it was killing them. They'll probably tell me I traumatised them for life."

These days, Gleeson's ex-pupils are more likely to be watching him at the cinema, but their opinion still counts. "I bumped into this guy the other weekend. I didn't recognise him, but he said, 'Brendan Gleeson. You taught me. Fair play to you, I always thought of you as a nice bloke." And I floated home, I absolutely floated home."

'The General' is on release