The opera singer Bryn Terfel still rides through the Welsh hills with Wagner blaring out of his car windows, he tells Arminta Wallace

'Hello there," Bryn Terfel says cheerily into the phone. "Aaar-rrr-UMPH," he adds without taking a breath, emitting a noise somewhere between a groan and a howl. This is impressive but also worrying. Terfel, who is probably the world's best bass baritone, makes no secret of disliking interviews. Can he be bored, irritated or annoyed before we've got to the first question? He is, it turns out, just watching snooker on the telly, and Ken Doherty seems to be on a winning streak. "The poor Welshman's not having a good day today, I'm afraid," says Terfel, although whether he's talking about himself or his compatriot Mark Williams isn't immediately clear.

Er, well, Doherty's compatriot inquires carefully, is this a good time to do an interview? "Oh, yes," comes the breezy reply. "Fine. Perfect, in fact, because it's an interval." And with little prompting, Terfel launches into a quick-fire summary of what he'll be singing at his recital in Dublin. Operatic arias, including a couple by Mozart, "a good staple diet for any singer". There'll be a guest artist: Catrin Finch, who has just been named royal harpist by the Prince of Wales. "And on top of having that honour, she's a wonderful harpist." Songs, of course: he has just finished a new - his Welsh intonation makes two clean syllables of the word: "nee-oo" - disc of collaborations, with pieces such as Danny Boy, Brahms's Lullaby and None But The Lonely Heart. "And then some Welsh songs, obviously, and then I finish with some lighter pieces from music theatre."

It's a wide-ranging musical menu for a singer whose first claim to fame was as the winner of the somewhat esoteric Lieder Prize at the Cardiff Singer of the World competition in 1989. But then versatility has always been Terfel's middle name. He has recorded everything from Welsh rugby favourites to Strauss - Richard, not Johann. And didn't he do an opera with Roger Waters of Pink Floyd? "Yeah, last year, and it was a fantastic eye-opener, I must say."

He adds: "I've always taken the opinion that whatever piece of music you sing, you have to sing it to the best of your ability. My career started with singing folk tunes in Welsh: very short, very beautiful pieces of Welsh folk tunes. Then it progressed to pieces with harp, pieces with piano. And there was the enormous leap of going to college to study music. I spent most of my time at the Guildhall [School of Music & Drama, in London\] in the library, catching up with classical music."

On stage, though, isn't he now being filed under "Wagner singer"? Does that surprise him or was it the plan all along? "Obviously a career goes in a certain path," he says. "I was thrown onto the stage immediately into principal roles, so that was me already on that road - and for young singers, of course, that means Mozart operas.

"So I did my Mozart time, and I've gone through my Strauss time, and I've gone through the one Verdi opera that I can sing, Falstaff. And, yes, it's true: for the next five years I will probably be engulfed in Wagnerian torrential waters."

It's an impressively laid-back account of a career that has seen Terfel acclaimed at the world's great opera houses and clock up a superb discography on the Deutsche Grammophon label. But does this musical progression mean he doesn't get asked to sing Mozart any more - he, who was such a memorably vivid Figaro?

"Actually, the Mozart operas have started to dry up, I must say. But, you know, I don't think I would sing Figaro or Guglielmo any more. You kind of grow out of that mould - and the young singers take over the mantle, just as we did from our predecessors."

Engulfed in Wagner means having to memorise huge chunks of German for the composer's four-hour operatic marathons. How does Terfel do it? "It's just like learning a piece of poetry. You have to be really tough with yourself. But I have one of these minds that work only when it wants to - and it tells me at 2 a.m. that this is the time to learn a song."

He chortles. "Very unsociable hours. But you have to go with that flow. Wagnerian learning - for me, it'll be repetition. Singing and singing and singing, and listening and listening and listening. Even in the car. That's how I tackle them."

So he rides through the Welsh hills with Wagner blaring out of his car windows? "Yes - what a wonderful image," he declares. And down the phone comes a robust bass-baritone version of the Ride Of The Valkyries.

Being Welsh is such a part of Terfel's identity that nobody would think of questioning it. He comes from near Caernarfon, he lives outside Cardiff and one of the highlights of his musical year is a summer festival he has been organising in those same Welsh hills for the past four years. "I always have a smile on my face when I talk about my festival," he says. "It's payback time for people who have helped me in my career and have been very, very generous, always coming to my concerts, wherever they are in the world."

Such dedication from his fans must be both reassuring and slightly alarming. Does he know what it is that people respond to in his voice? "No, I don't, really," he says. "But I'll tell you what I think is one of the most important things. One of my first international engagements was in Spain, singing a very small role in Samson And Delilah. It was just after José Carreras had got over his illness, and he wanted to try out Samson before singing it in Covent Garden, so he did it at an open-air festival. Now, due to the temperature, they begin their operas much later, so it was something like a quarter to one in the morning before the curtain came down.

"After the first night, I decided that I wanted my score signed by Carreras. There were nearly 600 people waiting for his autograph, but I decided I was going to get it by hook or by crook - and I was the last one in the queue. And Carreras was so gracious; he didn't hurry, gave everybody their due time. Finally he saw me at the end of the queue, and he said: 'What are you doing here?' And I said: 'Well, I take my hat off and I bow to you.' "

The lesson was not the adulation but the singer's response to it. "I'll carry that for the rest of my career. It doesn't take much for you to be nice to your friends, fans, whatever. To put people at ease, you know? It's all about enjoying what you sing and communicating it to other people. I think that's what comes from the stage, from my point of view: enjoyment. Even before I've opened my mouth I'll run onto that stage."

• Bryn Terfel sings at the Helix, Dublin, on Friday with the RTÉ Concert Orchestra