As Mark-Anthony Turnage tells it, the crucial turning point for him in becoming a composer came at the age of nine. He'd been learning the piano for about three years, and had been hating it. But "something happened when I was about nine. Something clicked, and I got really interested in classical music - Beethoven, Haydn, Mozart. I used to get very bored practising the pieces I had to do for the grade exams and I used to distort them and make them into other things. Obviously it was reasonably impressive because my parents were fooled into thinking it was me practising what I had to play at the exams. So, really, it was through improvisation that I began writing music".

Any lingering piano ambitions disappeared in his mid-teens, when he realised that there were "tons of better pianists than me, and if it was that overcrowded, then I probably wasn't good enough to make it". As for composing as a career, he simply didn't know how to go about it. He didn't think he could make a living writing concert music, so he sent a letter to the BBC Radiophonic workshop, which provides incidental music for radio and TV, and it brought an encouraging reply. "You know, `go do your studies, and we'll be interested to talk to you then'," he recalls. The message was: it can be done.

His parents were always very encouraging, but "they were religious, and I didn't really like that side of my childhood in a way. Sounds like I'm being very disloyal to my parents, but they were quite strictly religious at one point. To some extent, music was an escape from that as well".

The hint of rebellion this suggests is confirmed by the fact that he failed his first year at the Royal College of Music. The study of Bach chorales didn't appeal to him, in spite of the fact that "Bach is probably my favourite composer".

"I was already at the stage where I was writing fairly advanced pieces," he says. "I did find 16th-century counterpoint fascinating. And although I love Bach, I found these academic chorales very boring, all the stuff about consecutive fifths. Very dull. I probably stuck a load of dominant sevenths in, which probably irritated the examiner."

He had begun studying with the composer, Oliver Knussen, at about the age of 15. Knussen was faced with a paradoxical difference between the tastes of Turnage the listener and Turnage the composer.

"He couldn't understand why there was a gap between what I was listening to - which was Stockhausen, Boulez, Berio - and yet I was writing bad Wagner," Turnage says. "He couldn't work out why my taste was lagging behind. It was weird. He encouraged me to update my style." When the update came, it wasn't by increments; Turnage just leapt forward and became his own man. Opera, however, wasn't exactly beckoning to the fully-fledged young composer, known for his streetwise style and integration of jazz flavours into his orchestration.

"I wasn't that taken with opera, and I'm still not taken with a lot of opera," he says. "But I went to Boheme when I was 14 or 15 - I thought it was pretty good, from what I remember. But I wasn't completely bowled over, although I've loved it since. I didn't really see much opera. It wasn't the thing that my family did, although my dad actually sang - he's a tenor - and probably could have been a professional if he'd been from a different background. In fact, he had what's described as an `Irish tenor' voice. Singing was around, but opera wasn't. I don't think I thought, when I was in my late teens or early 20s, that I'd ever write an opera. In fact, I was pretty convinced I wouldn't."

What happened to change all of this?

"Hans Werner Henze," Turnage says. "I studied with him in Tanglewood in 1983. I hadn't seen any of his operas, although I'd heard quite a lot of his orchestral music. I knew he was quite an important theatre composer. For some reason - and I thought it was just a joke - he kept saying to me that he thought I would be good writing for the theatre, maybe not just opera but in the theatre. I just disregarded it. But he kept saying it, actually, and in 1986 he pulled me up. He'd started this festival in Munich, and said: `I want you to write an opera.'

"I did say: `No, it's not what I want to do.' I thought I was too young, for a start. Nobody would have commissioned me here. There was no interest here at all, to write an opera. It took somebody like Henze to take the risk - and he was taking an enormous risk. I'd been a music copyist before that, writing out parts. That was the way I earned my living for the first couple of years, apart from a few commissions. Then, with the opera, I was able to devote the next two years just to writing. I did very little copying 1986 to 1988. Henze commissioned Greek. That's how it started."

The initial reluctance, of course, soon turned to enthusiasm for "a proper commission for the first time, and it was going to be done in Germany, which I'd never even been to at that point". Henze had suggested Edward Bond as a librettist. But a delay in Bond's response found Turnage approaching Steven Berkoff, who proposed Greek, his updating of the Oedipus myth. The work, which is immediate and raw, was hugely successful. "I suppose because Greek really worked out well, and was widely taken up, it was like a second novel syndrome," says Turnage. "It took me ages to then want to write another one." And, of course, the search for a subject is never easy. It was in an Abbey Theatre production that Turnage encountered Sean O'Casey's The Silver Tassie. He regards himself as extremely fortunate in his relationship with librettist Amanda Holden. In reshaping the play, "she did make a lot of decisions for me. I thought she made great choices. Certain things, we just knew they'd have to be there".

Certain other things just had to go. There were, for instance, some references to the Catholic Mass in the second act.

"I was probably a bit paranoid that it would be a poor man's Britten's War Requiem. And Oh, What a Lovely War I was worried about as well. I didn't want to be any of those things," Turnage says.

The workshopping approach taken by English National Opera (ENO) for the premiere helped Turnage create what he wanted. He heard, with singers and piano, and later with orchestra, the music at every stage of the composition, and made changes. For instance, the chorus in the second act - the act "which most attracted me to the play in the first place" - was at first too simple, then too complex. It wasn't until he had produced a third version that Turnage felt he'd got it right. This act was "the hardest to write". He'd expected it to be the fourth, with all its dances. But, as it turned out, "that was probably the most fun, because of all the different levels". Even in the dress rehearsal, he made cuts to the two interludes, as "they just seemed the wrong length." It was nothing to do with the actual stage movement requirements.

Critical response to the premiere was mostly ecstatic. People seem to have wanted it to succeed for the sake of opera in general. The acclaim affected Turnage in an unexpected way. "I started questioning why I'd done it, which is a pretty strange thing to do," he says. "I went through a very bad period with it, to be honest, and I'm not sure if I'm totally out of that yet. I don't know if I should be revealing that in a newspaper. I worry a great deal when composers use emotional things to actually get an audience going. What I was surprised at was the emotional response people had to it in ENO. That really did surprise me. It was great, but it made me very suspicious of it. "To some extent I felt that I'd written better pieces than Silver Tassie and nobody had quite responded in the same way. I suppose I got annoyed about it. It became popular, it was really well attended, it's recently got awards and stuff. I wrote a really good piece four or five years ago and nobody gave it anything. You get very defensive for your older work. I was surprised at that. "I was worried that using the first World War subject is a way of getting people already emotionally charged up in the same way other people might use religious subject-matter. I became suspicious of the piece for a while. But then I had such a great time working on it that distance has made me more nostalgic for it.

"No composer likes every moment of their work. Most of my pieces, I only like bits; you like one or two minutes here or there. There are certain things in Silver Tassie that I'm really proud of - the duet between Harry and Teddy towards the end of the fourth act. I can't tell you why that worked out that way. You never can work out why that works, and other things you're less pleased with."

Opera Ireland's production of The Silver Tassie, directed by Patrick Mason, is at The Gaiety Theatre, Dublin on Saturday, Monday, Wednesday, Friday and Sunday. Booking at 01-4535519