For Fergus Johnston, composition began when he was aged about eight or nine. It was, he says with a smile, around the same time as he began to write poems. He's responding to a question about when music came into his life. "It all goes back, as I think all music does, to the parents," he says. "My mother always sang to me. I think that's the most important thing for any musician to do for their children, is to sing to them. For anybody, in fact. That's where it starts. Wasn't it Kodaly who said: `A child's musical education doesn't begin at its mother's knee, but at her mother's knee. She sings the songs that her mother sang to her'."

He has been told that around the age of three, when his family was living in London, he used to sit on the stairs outside the door of a woman playing the cello in the flat above. The sound of the cello, he says, has remained dear to his heart, but he never studied it. Nor has he got around to writing for it yet - no solo cello piece, no concerto - in the way he clearly thinks he someday will.

When he first started composing, however, he was studying the flute, so that's what his earliest pieces were for. He had tried the violin, but was terrified of his teacher, who seemed so old, so "skeletal". He was also put off by the fact that no one else could endure the sounds he made when practising. Later, he tried the French horn, which he never wrote for, because he couldn't play it.

At the age of 16, he took up the clarinet, which led to the saxophone, and that in turn led to him joining, at school, what he calls "a classical/folk/rock band". The band played what he describes as a version of progressive rock, in which "no two consecutive bars could possibly be in the same time signature". But although music was a passion, he hadn't actually been thinking of being a composer.

"I'm always reminded of Roger Doyle's comment that he's constantly amazed that he's a composer," Johnston says. "I wanted to be an astronaut, then I wanted to be a neuro-physiologist. Right through secondary school, I wasn't going to be a composer, although I loved music.

"There were two things I really cared about: science and music. Three things, actually. My mother used to complain about my hair as well; I had really long hair. So, my hair, science and music.

"The composition thing happened, I think, through finding a group in school who were interested in the same kind of music as I was, and who were also musicians, and we started writing things together. We had a sort of mutual appreciation society. I wasn't very good at harmony; I used to think linearly, and I still do. It probably comes from not being a piano player. But I used to like mapping out the structures of things.

"I'm very fond of structures, of creating structures, and I think that is where I got into composition - the combination of doing crossword puzzles, creating structures, uncovering the pattern that's in there somewhere. That's the way I write music, very often. It's like a game; it's a puzzle. Combine that with the music and you get composition."

When I ask about the shift from progressive rock to what he does now, he sniffs suspiciously around the word "shift". He muses on the relative merits of groups like Deep Purple, Yes (at one time his musical heroes), Genesis and Status Quo to explain what, as a teenager he had been trying to copy, before concluding: "I don't see it as a shift at all. I think it's just a move towards a more refined kind of complexity. That's the way I'd think of it."

He elaborates further. "I suppose my language evolved. Because, even working within a progressive rock background, the language is still quite simplistic. I wanted to find out more and I wanted to study music, so I went to Trinity.

"Another thing I haven't mentioned is I had a very strong interest in early music. My musical tastes were for very disparate sections of the musical canvas, if you like. And it was right through my teens that I was involved in early music. I used to play the recorder, and the crumhorn as well. I toyed with the cornetto, which was difficult.

"I went to study music in Trinity because I wanted to evolve, I suppose. Plus, the social aspect of the whole school thing disappeared once I finished school. That's when everything fell apart, there. Then, moving on to college, I got so much more, I suppose the word is . . . sophisticated. I lost my innocence, let's put it that way."

Losing his innocence involved learning about the world of 20th-century music beyond Stravinsky and Shostakovich. "I was very, very naive when I went into college," he says. "Most of my interest in contemporary music happened during college, and I realised, wow, what I'd been missing. There was just so much stuff out there that was new to me: Varese, Webern, Ligeti. It was a complete eye-opener."

Yet in retrospect, he says, what he was taught at college was "amazingly one-sided in terms of its concentration on Western musical thinking and its concentration on pitch at the expense of all other ways of looking at music".

When he finally got around to studying composition, it was with James Wilson at the Royal Irish Academy of Music, although in a way that was "off and on, because I sometimes couldn't afford the fees". Later, he was taught by the English composer, Bob Hanson, which involved occasional - because they were expensive - trips to Devon. The sporadic nature of the formal tuition has left Johnston feeling himself to be in the category of the self-taught.

"I've learned from studying scores, and I've learned from making mistakes," he says. "But I'm a very, very slow composer. I don't write quickly at all. Largely, because a lot of the time I think I'm actually re-inventing the wheel. It's something that I think you have to go through, because I think if you don't learn to do that kind of thing, you don't find your own voice. And I think I've started to do that now. At 41. At last. I think it takes that long. Very few people have a mature voice early on.

"I get these crazy ideas when I'm writing pieces. I model them on structures that I know and that I like. Piano Study No. 1 is actually modelled on an icosahedron."

I have to ask the number of sides in an icosahedron; it's 20.

"If you project it onto a flat plane," Johnston continues, "And take the slopes of the angles as being the different tempi, and the different panels of the icosahedron as being the different sections of music, you can follow a course around the flat, projected surface of the icosahedron in that piece - if I tell you how to do it. I think it's a minute and 11 seconds long. I wrote it for Raymond Deane's 40th birthday. I wanted to be able to say to Raymond Deane that I gave him a virus for his 40th birthday, because a lot of viruses have icosahedral shapes. He didn't like the title, and I can't say I blame him. So I changed it."

A couple of years ago, Johnston went back to college to do a post-graduate course in music technology at TCD.

"I felt I was becoming a bit of a dinosaur, and being left behind by up-and-coming composers who had all the chintzy, glitzy attractiveness that technology brings to their music," he explains. "I found myself a student of a much younger composer, Donnacha Dennehy, which was rather interesting.

"I had to do it because there were pieces I had been writing according to algorithmic principles, which I had worked out with a calculator. It was really slow. Now that I've done that course, I've got access to a whole load of software that will help me do algorithmic composition. I thought it would speed things up. Actually, it doesn't. It just gives you a whole lot more material which you have to sift through and throw away."

Something he hasn't thrown away is what he calls "a great idea for a cello concerto", which he has been nursing since 1990. And a selection of his chamber music in the Hugh Lane Municipal Gallery's "Sundays at Noon" series on February 4th includes Opus Lepidopterae, Signals, Bogboogie, Psyche, Journeys (music from an installation created as a project with recovering drug addicts), The Oul Winda Rag (one of the test pieces for the 1997 Dublin International Piano Competition) and Morrighan, a recent work for baroque flute and harpsichord, with live electronics, another new development from his return to college.

There's also another electronically-flavoured work in hand, which will be premiered in conjunction with the restoration of Ardfert Cathedral in May. His biggest worry there, however, is not musical. The venue doesn't have a roof. And Irish weather still remains beyond the control of both music and technology.

Eleanor Dawson (flutes), Jane Chapman (harpsichord, piano) and Izumi Kimura (piano) play the music of Fergus Johnston at the Hugh Lane Municipal Gallery of Modern Art, Dublin on Sunday, February 4th, at 12 noon. Admission is free. Full details from 01-874 1903.