The ascent of Donnacha Dennehy has been rapid and steep. Three years ago, at the age of 27, he was virtually an unknown in his native Dublin. Today he has a string of major commissions to his name, is one of the founders and artistic directors of the Crash Ensemble, and his work is heard regularly abroad as well as at home. In one of those strange, post-colonial adoptions, he has even been featured in one of the London Sinfonietta's State of the Nation weekends, planned as a showcase for young British talent. And, unlike the similarly-featured, Belfast-born Deirdre Gribbin, who has lived and worked in England, Dennehy has consciously avoided studying or working in Britain.

So what's the background of this voluble, charismatic character, who has the air of being a maverick while also clearly being at the heart of the new music scene in Ireland, with his own performing ensemble and commissions from the likes of the RTE Vanbrugh Quartet, the National Symphony Orchestra and the Music Network to prove it?

As musical children go, he was a late starter. He was nine when he had his first music lesson. "There were recorder lessons offered at school by this woman called Miss Glynn. I remember both being entranced with the recorder and with her. She was my first music teacher. I loved it immediately, and I started writing immediately, too. My first piece, aged nine, was a Symphony, lasting 21 minutes, for solo recorder. Then lots of pieces like Off to the Mountains. It was my pastoral period: the only period in my life that's been pastorally influenced."

But before that, there had been his transistor radio, acquired at the age of six, and used for listening to broadcast Mass, with its singing and chant, pop music, and a classical music show, which he says was probably Music for Middlebrows, from which he used to tape his favourite bits. But alongside all of these, there were also excursions to the Dublin Festival of 20th Century Music. "I used to attend it religiously. I was probably one of the youngest attendees. I knew all these guys like Raymond Deane, Roger Doyle, Gerald Barry when I was a kid. I knew their music. I went to see Stockhausen when he came in the year of the big snow [1982]. At an early age I was massively into new music."

The key memory that has stuck in his mind is actually of his father, always a source of encouragement, getting him to listen to a piece on the radio that had sirens in it. "Later, I recognised it as being Varese's Ionisation. This captured my imagination. Actually, this was the first capturing of this idea of urban music, which I've since been fascinated with."

A bad bout of nerves at the Feis Ceoil - he had graduated from the recorder to the flute - taught him that he didn't really want to be a performer, and, he says, "There's a resonance there with Glenn Gould's thing about live performance being the last of the blood sports, for the audience, that is. After school, I decided to go for Trinity. And that was the biggest opening in my life; from there, I just never looked back."

At college, his works were played in student events, the college orchestra set up by Fergus Sheil, a festival of new music, set up with Gavin Kostick, the playwright. "I never made any attempt to have my music performed by the symphony orchestra, or anything like that. Because, while in Trinity I was working on elements of craft. This was not necessarily my own voice, at all. In many ways, my voice was essentially very much closer to what I was doing as a young kid, actually. But I didn't have the technique to be able to realise it properly, then. My development only really got going in America."

At Trinity, he was strongly under the influence of Bach. "I had a wonderful counterpoint teacher, Joseph Groocock, who I almost consider one of the most important contributors to my technique, even though you'd hardly recognise it in the way it rears its head now. He was completely opposed to modern music. Hated it. He took me aside before I left for America, grabbed my arm, and said `I hope you won't write any of that bang, crash stuff in America.' I said, `I probably will, that's one of the reasons I'm going.'

"I did really love Bach, and still do. I was also very interested in Ligeti. He was probably my favourite living composer. Without a doubt, actually, I love his piano Etudes, his Chamber Concerto. There was that element of super-craft in his work, yet there's this absolute panache in the way he delivered it." Ligeti was for Dennehy the summit of pattern-oriented music. "There were also people like Reich, who I liked as well. And I quite liked Arvo Part, who I went on to dislike, because some of the stuff was too . . . soft-porn-like. Some of it, though, I still kind of enjoy. Composers who didn't figure much in my imagination then, but who since do, are people like Stockhausen, Cage, Feldman, people who are less easy to place in an organic tradition than someone like Ligeti, or Lutoslawski."

Encouraged by Hormoz Farhat, TCD's professor of music, to go off and expose himself to new influences, Dennehy went to the University of Illinois, in Urbana, Champaign. The place seemed like a nightmare in the middle of nowhere, but musically, he says, going there "was the best decision of my life so far. Where I had been used to discussions about music nearly always taking as the assumption that music is going to be organic, or something like that, or that the piece of music demonstrates itself, here, suddenly, I was in the midst of 12 composers on the faculty, all with hugely different ideas about what music should do.

"After all, Illinois was the place that housed John Cage as a professor for a while. Harry Partch worked there. It was a hotbed of experimentalism. There were still strong figures there when I arrived - many of whom are dead now or left. Sal Martirano, who was a huge guy in multi-media work and music theatre. Herbert Brun, who'd come from Cologne, indeed was the first person to put up Ligeti - or so he claims - when Ligeti came to the West. He used to say, `You must write the music you don't like yet', and he ran classes in experimental music that went from 10 to midnight. I'd never had a class before where you broke for red wine halfway through, and sometimes had caviar, even - and Brun was a hard and fast communist.

"It was fascinating, the rows that would erupt in the composers' forum, the walkouts during concerts, people having to defend what they were doing, massive divergences - people who would never write a piece by hand, there always had to be processes of computers that would do it, others who would never use a computers, others who believed if there was even one, single reference to another piece of music, or a chord or anything, the work was completely nullified, and others who went in the opposite direction, everything had to be references. Massive arguments. It was an absolutely enlightening time for me. There I was, in a poky, mid-Western town, and it was the biggest opening-up of my life. I still feel nostalgic about Illinois."

From Brun, "who knew Adorno, studied with Wolpe and also knew Feldman", Dennehy got a sense of historical connection, "right back to the Second Viennese School". Through Sal Martirano, he came to value his own sense of intuition, and took encouragement in a "go-for-it approach". And from William Brooks, who, he says, was always very clear-thinking, he learned about thinking through everything he did.

It was in Illinois that he developed his confidence in risk-taking, and a feeling of security about implementing his own ideas about music and composition, however non-standard they might be. "Composition became a much more enjoyable activity, great fun, super fun, which I still have."

The Crash Ensemble with guest pianist, Joanna MacGregor, are at the Project Arts Centre tomorrow night and Sunday, for a concert billed as Happening. It follows John Cage and his influence, with works by Cage himself, Django Bates, Andrew Toovey, Deirdre Gribbin, Kathy Hinde, Stephen Montague, Talvin Singh/Joanna MacGregor, Howard Skempton, Philip Glass, Steve Reich, and Frederic Rzewski. Information and booking at 1850 260027.