`Sometimes I see it written down, and it seems so strange: Benjamin Dwyer, composer," says Benjamin Dwyer, giving the froth on his cappuccino a thoughtful stir. He'd better get used to it. His name will be all over the programme for this evening's gig at the National Concert Hall; two pieces by Dwyer the composer, with Dwyer the classical guitarist also putting in an appearance, as guest soloist in his own Concerto for Guitar and Strings.

But it's the Concerto for Percussion and Orchestra which, if we lived on a planet where people actually got excited about world premieres, would grab all the headlines: a brand-new concerto on a grand scale featuring an enormous orchestra and some three dozen percussion instruments.

"It's huge," says Dwyer, grinning happily. "It's nearly as big as the thing they did by Boulez two weeks ago where they had to extend the concert hall stage. They've since taken the stage down again - I'm obviously not as important as Boulez." But he was delighted with the commission, one in a series of works which RTE is commissioning from Irish composers for the principal players in the National Symphony Orchestra. "I didn't seek it out: Richard O'Donnell, the NSO's principal percussionist, wanted a concerto and he chose me. But it's a great concerto to be offered, percussion. The jewel in the crown of modern concertos."

It's not all thumps and crashes and weird bits and pieces, though. Percussion may be the magic toybox of the orchestra, but it can also induce a host of logistical headaches. "Take marimbas. Different orchestras have different-sized marimbas. And I'm using bass bells, each of which is 11 feet tall. At the time of composing there were four, but they've since acquired more: and I would have had more, if I could. And percussion instruments are not standardised. You can write something which is correct according to the manual - only to discover that the instrument which is available in reality has three or four notes missing."

Then there's the business of arranging the instruments so that the soloist can get around them all at top speed without tripping over an 11-foot bell. "I think Richard is going to be running around the stage. It might," he says, with commendable understatement, "be quite dramatic."

The piece is structured around the Sanskrit concept of Prakriti, or Nature, with its three contrasting phases Rajas, Sattwa and Tamas - in a nutshell, Creation, Sustenance and Destruction. "Whichever is in force at any given moment determines how nature is acting - for instance, I think we're going through a Tamas phase in terms of the ecological situation at the moment," says Dwyer. It is, he says, a simplified view of a complex philosophy, but it is a theme which has fascinated him since he wrote his opus one, a solo saxophone piece, in the early 1990s, and it provided the perfect framework for the new concerto.

"The first movement is just cellos rippling, pianissimo, then it develops to use the whole orchestra. I decided to use only non-pitched instruments for the outer movements, so the first movement is largely drums, toms, gongs and cymbals. The second is more melodic - at least, it has its moments - and I use vibraphones, and the perfect fifth interval, which doesn't imply anything, just sustains itself. The last movement is - well, I wanted it to sound quite barbaric. Then I added a prelude in which I get the whole orchestra to hum." Yes, he did say "hum". It's to do with the Hindu philosophy of consciousness. Don't ask: just get along to the Carolan Room by 5.45 p.m. for Kenneth Edge's pre-concert talk, which will explain all. "But then again," adds Dwyer, "I would hope that even if people didn't know what it was about, they could just enjoy it as a piece of music. I hope it's not so programmatic that it requires a story."

The story of bringing a new piece of music into the world is often something of a saga, and this concerto has been through the usual process of bargaining and negotiation. There's never enough rehearsal time for new pieces - and in this case, there isn't a big enough orchestra to play it, for the size of orchestra it requires simply doesn't exist in Ireland. Dwyer accepts these as "teething troubles" - but was clearly a little miffed when one conductor - not, he is at pains to stress, the man who'll wield the baton tonight, Nicholas Kok - refused point-blank to have any truck with his pristine handwritten score. "And I've a fairly neat hand, too - I come from the Queens stable," he says.

"I did a PhD at Queens, and they are incredibly strict about your p's and q's in terms of notation. I mean, the piccolo note at the top of the page has to correlate visually to the double bass note at the bottom, or you're in big trouble; and when you're writing your scores, the pages all have to be done in pencil first. Like the monks doing the Book of Kells." The parts have now been published by RTE.

Dwyer's musical training was all performance based, and he emerged as a fully-fledged classical guitarist, ready to perform Rodrigo's fearsome concerto at the drop of a hat. So why did he get into composing? "Ken Edge said, `I'm looking for a piece - can you write me one?' And I said, `I don't know.' And he said, `Well, look, you write me one, I'll play it'. That was the solo saxophone piece I spoke about earlier.

"That's how I became a composer. But I just consider it a natural extension of being a musician. A number of my composer friends have never played a note of music in their lives - and I have a slight problem with this, because it seems to me that they've decided to become `composers'; yet throughout the history of music you had "performers who wrote music". Schumann was a virtuoso pianist; Bach, Scarlatti, Mozart, Beethoven. Haydn conducted; so did Webern, Mahler, Stravinsky. And Boulez: a stunning conductor. There are one or two exceptions, obviously, and everything in life is becoming more compartmentalised, I suppose. But I can't help feeling that something gets lost during that process of compartmentalisation."

The programme for tonight's concert is very much a reflection of the performer-as-composer idea, opening with Dwyer's concerto for guitar and strings, which was originally commissioned by the Instituto Cervantes to celebrate the centenary of Lorca's birth, followed by Arvo Part's In Memoriam Benjamin Britten. "I'm not mad about Arvo Part - I get bored with that religious minimalist stuff - but I did my Master's degree on Britten, so I've a particular affection for him, and this is a gorgeous piece for strings which finishes with one little bell." And sets the scene nicely for the percussion concerto? Dwyer grins again. "Tenuously. But still. Let's see how it goes."

Tonight's concert in the Horizons series begins in the National Concert Hall at 6.30 p.m., and admission is free. The preconcert talk is in the Carolan Room at 5.45 p.m.