Cillian Murphy's piercing gaze has featured in films such as Batman Begins and The Wind That Shakes the Barley; now he has set his sights on the theatre. Jasper Rees meets him.

Cillian Murphy is the bashful owner of the most dazzling sky-blue irises since Peter O'Toole's lit up the Jordanian desert in Lawrence of Arabia.

"It would be foolish to become self-conscious about it," he says, "because I just see them as for looking through. It's not anything you can ever think about."

So film directors tend to think about them for him. Danny Boyle shoots them in close-up in both 28 Days Later... and the forthcoming sci-fi thriller Sunshine. Wes Craven beams in on them in the plane drama Red Eye.

As the Scarecrow in Batman Begins, he at one point wears a sack over his head with peepholes, echoing a scene in the ensemble film Intermission, in which Murphy wears a monster mask to disguise himself from his ex-girlfriend. The scene has only one flaw: she can still see his iridescent peepers.

If the eyes have it, they currently face competition from a ragged and startlingly ginger beard that Murphy has grown for a new play that reunites him with the director of Intermission, John Crowley.

"I had always wanted to work with him on stage because I thought his work was incredible," says Murphy. "When I came to London as an actor who had done nothing, I remember going to meet John and just asking for advice. He was very supportive. But it had to be the right material."

The right material turned out to be Love Song, a new play by John Kolvenbach, whose On An Average Day provided a hit for Crowley and Woody Harrelson four years ago. The beard belongs to a character called Beane, who, when we meet him, has an ingenuous, almost autistic lack of irony and disconnection from all social norms represented by his sister and brother-in-law. His world view changes when he meets a beautiful stranger.

"It sounds corny," says Murphy, "but when you're in love, everything from the cheesiest Phil Collins love song on the radio to a turkey sandwich can be the most incredible thing in the world. My character goes from his world shrinking to falling in love and everything becoming vivid and Technicolor. Actors love to make that transformative leap."

Murphy, 30, made his first transformative leap in his late teens. Growing up in Cork, he had been in bands "for as long as I can remember" and had embarked on a law degree, "which was without doubt the wrong way for me to go."

He had what he calls "an epiphany" at a production of A Clockwork Orange in Cork City. "It was in a nightclub and it was promenade. It was totally removed from the proscenium arch and eating sweets. That's what made me go and ask them, 'Can I be in a play?' I had no idea that it would ever turn into a career."

The play he successfully auditioned for a year later was Disco Pigs, Enda Walsh's portrait of an intense, symbiotic love affair between two Irish teenagers. As it won awards and travelled beyond Ireland to Edinburgh, London, and Canada (and was later made into a film), Murphy found himself a professional actor.

"I assumed I would be working in theatre and then maybe get a part in film here and there. That's what I hoped for, and it's worked out a bit better than that."

He worked in theatre for four years. It was only after Boyle cast him as the lead in 28 Days Later... that directors started to notice. He was cast as a creep or two, but Neil Jordan with Breakfast on Pluto and Ken Loach with The Wind That Shakes the Barley were both persuaded that he could be the moral heart of a story.

There are a lot of film stars treading the boards these days, but few in quite the prime of professional life that Murphy has entered since he last acted on stage three years ago in an Irish touring production of The Playboy of the Western World. Why go back?

"You could work all the time in films, but the bar would drop significantly. I have tried to be patient and choose very, very carefully what I do in film. It was a choice between doing a not-great movie or doing a wonderful play. It's an easy choice to make."

His sense of what constitutes a great movie has been sharpened since working with Loach on his drama about the factions within the Republican army in the early 1920s.

"That was a stand-alone experience. None of the superlatives that you draw on when talking about Ken Loach quite seem to hit it. He is all the good things you read about." (Typically, alone of Murphy's directors, Loach didn't make a big thing of the blue irises.)

That's not to say Murphy feels remotely apologetic about his foray into Gotham City. "I was terrified because there is a history of Batman villains being pretty amazing. I was a huge Batman fan. I had a Batsuit, I had a Batmobile, I watched all the movies, all the cartoons. All of a sudden, I was in that world and it was very hard not to react like a 10-year-old child."

He has also had to contain his excitement in Love Song. In a cast that also includes Kristen Johnston, best known here for her outlandish performances in Third Rock From the Sun, and Neve Campbell of Scream fame, is Michael McKean, who played the feather-mopped lead singer of the spoof metal band Spinal Tap.

"I'm in a play with David St. Hubbins!" says Murphy. "I have to be careful not to bring it up every day."

Out of respect for the master, he declines to discuss his own musical output, other than to say that Sons of Mr. Greengenes, the school-of-Zappa band he fronted, was offered a record deal he doesn't regret not signing. It's hardly surprising that the next best thing to rock stardom has also been dangled in front of him.

"I've been approached to play a number of dead musicians, and I've turned them down each time because I feel it's a hard thing to pull off. There are few rock biopics that have been successful. I just think it's risky territory. It's very hard to catch the sense of what a group was or to catch the personality or spark, and then you end up doing an impression of somebody, which to me is not interesting."