An Arditti Quartet concert is not something to be written about, but to be experienced. It's difficult to convey in words any true impression of the controlled explosiveness they command, the outcome of a very particular dynamism and energy, harnessed in the pursuit of intricate demands of rhythm and articulation. Nor is it easy to communicate the astonishing, apparently ever-expanding range of tone, colour and texture they've developed.

But most of all, it's hard to convey the sense they generate in performance that everything they play is music that matters. The concentration of their music-making, the clear mastery of often unrelentingly extreme demands, tells everyone how much the music matters to them. The miracle of their achievement is the way they make it matter to listeners, too.

The quartet's special place in musical life was acknowledged earlier this year when they won the Ernst von Siemens Music Prize, worth 250,000 deutschemarks, or just over £100,000. It's the first time this award has gone to a group rather than an individual, and also the first time it's been awarded to anyone specialising in contemporary music. First violinist Irvine Arditti formed his quartet 25 years ago, specifically to play contemporary music. "I suppose because it was my hobby. My interest was to listen to new music from my early teens. There was the possibility in London to hear interesting people - composers like Boulez, Stockhausen, Berio and others. I became fascinated with this music and wanted to form a quartet that was exclusive to it. After a little while, we became exclusive to the 20th century, in order to have some context on new music, which I think is also important."

The fascination of the new is something that's grown over the lifetime of the quartet. "Today, there's so much variety and depth of new music. You never know what's going to be flung at you next."

The next-longest-serving member of this international quartet - the members are an Englishman, an Australian, an American and a Sri Lankan - is cellist Rohan de Saram. "Contemporary music is, to my mind, the most natural thing. I think it's right that one should play quite a lot it. In the time of Haydn and Mozart it was almost all contemporary. Things have become more and more backward-looking."

Second violinist, Graeme Jennings, sees the present as one of those times when composers have shown a particular interest in writing quartets. He compares it to the classical period of Haydn, Mozart, Beethoven and Schubert, and the early years of this century when Schoenberg, Berg, Webern and Bartok accorded it a special place. "I think now, at the end of the 20th century, the string quartet has proved to be very fertile, and many people have come to it. And I think this group has been one that's particularly been behind that rebirth of interest in the medium. It's a very exciting moment to be playing the music of our time, the string quartets of our time." And, given the Arditti's close involvement with the composers who've written for them, they have in a sense contributed to the defining of music in our time. Irvine Arditti takes up the point. "I think it's probably clear that we created a major interest in the string quartet. I think perhaps it's taken this group to redefine the idea of quartets in the second part of the 20th century. Before then, there were groups that played and influenced some contemporary music. None of them played anything like the volume of our repertoire." Remarkably, the Ardittis have brought their very special advocacy to around 700 works.

The viola player, Dov Scheindlin, also feels "it is a very special time to be playing in a string quartet and to be playing contemporary music in a string quartet. There's something about the medium that's really satisfying for string players - in music of all periods. Think of the parameters of the quartet: four players playing instruments of similar construction with a wide range. First of all, composers have to boil down their language to the very essentials. Without the possibility of flashy orchestrations, they have to decide what ideas are really important to them. Many composers have written their best and most satisfying works for the string quartet, both in old times and now. "For a string player, the quartet gives you both individuality and yet a group homogeneity. It's uniquely musically satisfying, rather than being in an orchestra where your voice is very much blended in with so many others, or, at the other extreme, soloistic playing, which can be such a lonely affair. This is really the ideal kind of medium, in which you can contribute and yet feel you're part of something larger."

With a specialised repertoire and a heavy work schedule - daily rehearsals, and concerts spread over 10 months of the year - there's not a great deal of time for other work or other types of music. "We play some of what I would call classical pieces in the quartet," explains Arditti, "like Bartok, Janacek, Ravel. Although it's not Haydn or Mozart, it's for us a mostly different sort of playing to what we do with new music. We're not only playing the latest new music." For him, the single most stimulating aspect of the quartet's work is the contact with composers. "The idea that we are the interpreters, the direct link between the music on the paper and the sound that comes out. That we are creating things, the way we do it, the way we lay it down, the way we perform it - first perform it or develop our interpretation and then record it - is the way other people will listen to it and think, it's handed down by these people and that's the way it should go."

"The whole point of the quartet, right from the very beginning, was to work with the composers, and mould an interpretation from the way they wanted it. For me that's stimulating and fascinating. Meeting the composers. Asking them questions. You can't do that with Mozart. Also, you never know what you're going to find. It's like a Pandora's box."

The members see themselves as having an ambassadorial role, in taking particular music to countries where it's never been heard before, and also in working with young composers. "In a way, teaching them how to write for the quartet. How can you teach them? You play the music they've written, show them how good or how bad it is. Play what they've written."

From 1982 to 1996, the group taught at the Darmstadt summer courses in Germany. Now they have no formal connection with education. "The quartet has NEVER" - and you can hear the indignant capitalisation as Arditti speaks the word - "participated in any institutions, teaching or any sort of capacity in any of the colleges in England." Purely because they haven't been asked. The quartet are based in London, but home gigs don't feature prominently in their schedule. However, the British Council, which has made the Bantry residency possible, has always been a staunch supporter. "They told me a few years ago that we were the most travelled group," says Arditti. "Which doesn't surprise me, because we only travel. The British Council has always supported us. Inside of England has been rather sad for us over the years." And Jennings can see a certain benefit to the work pattern. "There's a sort of intensity of focus you get when you're touring together and playing together as well." Hand in hand with the intensity goes discipline. "We prepare very much before the first rehearsal," explains Arditti, "so that we're not swimming." Begin as you want to proceed. Get it right. Yet there are exceptional freedoms.

JENNINGS adds: "One of the nice things is that we are coming to a brand new piece with a clean slate. No pre-conceived ideas. No one's heard a previous interpretation, or played the piece with another group, like you often have in normal classical chamber music situations. When we come along, the interpretation is something we arrive at collectively."

"We learn another discipline within this quartet," says Arditti. "Whatever piece we're playing is the best piece of music ever written" - "At the time you're playing it," Jennings interjects. "Yeah. Thank you. You have to give your all. A bad piece is much more challenging for a performer - to put it over, to try to make something of it to an audience - than a good piece. You can get away with playing a good piece badly, and you'll still hear that it's a good piece, if maybe not a good interpretation. "I think we think collectively. I'm not saying we think the same way, but we sort of complement each other in our working process. It's so useful when you're working with people, to have someone telling you what you're not doing right. It's not always so easy to know that when you're playing difficult things. We're all contributing to every rehearsal."

The Arditti String Quartet are in residence at the West Cork Chamber Music Festival from June 27th, to July 3rd, playing works by Thomas Ades, Beethoven, Gyorgy Kurtag, Gyorgy Ligeti, Schnittke, Iannis Xenakis, Brian Ferneyhough, James MacMillan and Schoenberg, and giving master classes

The West Cork Chamber Music Festival runs until Sunday. Participating groups include Quatuor Mosaiques, the Parnassus Trio and the Vanbrugh String Quartet, and master-classes will be given by pianist Joanna McGregor, fiddler Martin Hayes and singer Patricia Rozario. Details from 027-61576