THE ARTS:More than 40 years after releasing In C, Terry Riley is still trying to strip music down to its purest essence, he tells Michael Dervan

Terry Riley's In C is the first classic of minimalism in music. It was premiered in 1964 when the composer was 29. Alfred Frankenstein reported in the San Francisco Chronicle that the composer had "developed a style like that of no one else on earth, and he is bound to make a profound impression with it".

The style was "formidably repetitious", but the outcome was positive. "At times," wrote Frankenstein, "you feel you have never done anything all your life long but listen to this music and as if that is all there is or ever will be, but it is altogether absorbing, exciting, and moving, too."

Riley, who's now in his early 70s, sports a beard that would make for perfect casting as Albus Dumbledore in a Harry Potter movie, and radiates a guru-like, authoritative calm. I caught up with him in his studio in Richmond, California, a room crowded with equipment, computer screens and microphones, and dominated by the presence of a grand piano. It's the studio of a hands-on composer, a man who likes to play and play with music as much as he likes to write it.

It's more than 40 years now since In C was first heard. But the piece still dominates all discussion of Riley and his work. It can be played by "any number of any kind of instruments". The score consists of a single page of 53 musical fragments or modules which the performers play through, repeating each ad libitum, moving on as they see fit. So, while they co-ordinate in pulse, they progress through the material at different rates, and no two performances will ever be quite alike.

Riley grew up in Northern California, where it was "kind of hard to get access to good teachers", and he was in his late teens when he "really started to deepen into music". He absorbed everything he could, especially in the area of 20th-century music, from Schoenberg and Webern to Stockhausen and Boulez. And at times he kept the wolf from the door by playing ragtime at San Francisco's Gold Street Saloon.

He bypasses mention of his formal musical training (at San Francisco State University and the University of California in Berkeley) when he describes the genesis of In C. "I was basically trying to educate myself, and I was trying to write pieces like the ones I heard. Like a typical student, I wrote in many, many styles when I was young, which is kind of a good training. I was mainly self-taught, I wasn't taking any theory classes, things that people do like counterpoint, so that shaped a lot the way I started to do music. I started doing it before I really knew much about what I was doing, everything I did up until In C.

"And the reason I think In C is a very unique piece, is because of that background. I didn't really use a model for it. It had no model." He had been working with tape recorders, and had developed a specific interest in tape loops and repetition. Repetition, he says, "really appealed to my nature", and "part of it, frankly, was, that it was also the period of psychedelic drugs. The psychedelic experience is one where a little event becomes a major event in your experience. If you've taken LSD, a mosquito on the wall could become a dinosaur.

"The tape loop was a static experience, in the sense that you have one event that's being repeated over and over again. But in the repetition it reveals a lot of the nature of what that musical material is, because you're hearing it over and over again, you're becoming very familiar with all its contours. So, even though you're hearing the same event, it's not the same. It's changing. That was a philosophical bomb for me. It really woke me up to something I hadn't experienced in music before.

"It had happened in music before, because musics from all over Asia, Africa and the Far East worked with this principle for centuries. They were all minimalists. They took a certain pattern and they repeated it over and over again to build up a sense of ecstasy. It wasn't new for the world. But it was new for the particular niche I found myself in as a composer of modern music."

He wasn't, he says, reacting against more complex or dissonant musical styles. "It wasn't a reaction against, for instance, the dodecaphonic style that was happening in Europe. I did like that, but it didn't fit me. I still enjoy it. I admire the people that did it. It's just that, as you're growing up, you have to find what really moves you, what you can live with every day. I wanted to do music all day, every day."

Unlike other major minimalist composers, Riley seems to have shed any serious discomfort he had with the description minimalist. "There was a time when I felt it was very limiting, and when people applied it to me I thought it had the connotation of being simple-minded, that if you don't know how to do real music, you do minimalist music. But in a certain sense I think it's justified in my case because I do have a real love for stripping things down to their skeleton form, and then starting up again. I think that's essentially what minimalism is. It works with very basic forms."

HIS CAREER AFTER In C has taken him in rather unexpected directions. In 1969 CBS brought out an album, Rainbow in Curved Air, which featured one-man-band improvisations, using looping techniques to create layers in the sound. Its effervescence and sweet cascades were much admired and taken up in the world of popular music. Shortly afterwards he abandoned composition to pursue the study of Indian music, and during the 1970s his activities were as an improviser rather than as a composer.

"I had always been more interested in playing and improvising than sitting down at a desk and writing out a piece. I'd always found it more fun to play, and the other a little bit tedious. I always had trouble with the decisions. Like, if I'm playing it, I'll play it 500 different ways, but it's hard to write it down so that it can be played 500 different ways. In C solved that for me in a certain sense. But I haven't been able to solve all my pieces that way. In C just happened to be the kind of piece where you write it down one way but it can be played a million ways. You can't do that with everything.

"In 1970 I met Pandit Pran Nath, the great Indian master vocalist, and it transformed my life. Once in a while a musician will come along who will totally blow away everything you thought before. He opened up this road to me, of Indian classical music and his own particular genius in it, which was very unusual even for India. I went back to India with him, and I started this very serious study of Indian classical music.

"For the next 10 years I absolutely didn't write anything at all. I was going to India a lot, and I was composing a lot of pieces, but I didn't write anything down, and didn't write anything for anybody else, until about 1980, when I met David Harrington of the Kronos Quartet."

The group was young and daring, and Harrington pulled a fast one by advertising the performance of a new work which Riley then felt obliged to provide. From 1980 onwards notated works have continued to flow, including a two-hour string quartet cycle, Salome Dances for Peace (again for Kronos) and even some orchestral music, a commission for the centenary of Carnegie Hall in 1991 coming as a total surprise and providing "one of the most bizarre experiences of my life".

It took some time to adjust to the world of the orchestra, and the Carnegie commission, Jade Palace, has "a lot of great music, but not a great sense of what to do with an orchestra. It was clumsy."

Riley's Irish debut this weekend is a multiple homecoming. He's visiting the land of his ancestors for the first time, and he's also revisiting the world of the Táin Bó Cuailgne, which he explored in his imagination to write the 1987 saxophone quartet, Chanting the Light of Foresight. He'll be improvising on piano, playing with his guitarist son Gyan, and performing with both the Arte Saxophone Quartet, and the Crash Ensemble, who'll be playing both In C and a work specially written for the occasion, with the long and provocative title, Loops for Ancient-Nude-Hairy-Warriors Racing Down the Slopes of Battle.

It will be an opportunity for Riley to do what he likes best. "I like working in small ensembles. I like the intimacy of improvising with a few musicians who are really well trained and listen to each other. To me that's still the most satisfying."

The Louth Contemporary Music Society in association with Drogheda Arts Festival present Terry Riley: Spirals of Ragtime and Raga, with concerts in Drogheda on Friday and Sunday, and workshops and a composer interview on Saturday. Booking and information at 041-9876100 or www.ticketmaster.ie