A short, slightly built chap, who shuffles in with a zipped-up combat jacket and combed-down hair, as if he has just come off stage after a bum gig at the Hacienda, Murphy might as well be the new Colin Firth for all the physical resemblance. But, hey, he and Farrell are Irish, right? They are both actors. Lazy, isn't it? On a sofa at London's chic Charlotte Street Hotel, Murphy grabs a cushion and clutches it to his chest like some kind of question-proof vest. Not that he is an ungracious interviewee, just guarded.

With good reason, it would seem. Life was chugging along nicely until his starring role in last year's smash horror flick, 28 Days Later, found him suddenly spread-eagled in the spotlight of publicity. Now everyone wants a piece of him.

It's all getting a little out of hand. On the internet, Murphy fan sites are multiplying; elsewhere, biographies and profiles run unchecked. According to several of them, he is a keen rugby player. "Look at the size of me, man," he says dismissively, rolling his eyes. "I love acting, I love the whole process, I love being on stage, I love making films -it's the other stuff, like interviews and things, I don't really care for. I find it quite uncomfortable speaking about myself. I still haven't figured it out." Oh dear.

Murphy's new film, InterMission (or "the Irish Amores Perros", as it is being pegged), broke box-office records across the water this summer and is about to open here. Featuring Shirley Henderson, Colm Meaney and David Wilmot, this ensemble comedy marks the directorial film debut of the theatre veteran John Crowley. Murphy plays John, a sort of confused misfit ("He's just a bit crap"), who loses his supermarket job and superficial girlfriend (Kelly Macdonald), only to end up embroiled in the schemes of a local hood. Just to complicate matters, said hood is played by Colin Farrell. Back on the old sod, InterMission has been nominated for a hatful of domestic awards. The humorous nature of the film is undercut with a mean-spiritedness that might not be everyone's cup of tea (especially if laced with HP Sauce, a running gag throughout). But in the wake of films such as On the Edge, it shows Murphy's preference for unsettling characters and black humour. "People laugh at funerals, don't they?" he says. "I'm always drawn to darker stories - people that aren't completely evil or completely good. To play that ambiguity is interesting."

Murphy grew up in Cork (though he now lives in west London) and is older than he looks: 29, though he could pass for 19. With his clear blue eyes and delicate features, the camera loves him, but his passage into acting was serendipitous. He abandoned a law degree when the band he was playing in proved of greater interest than tort tutorials ("My parents were none too pleased"). Then, on a whim, he went to audition for the play Disco Pigs and bagged the part. When the production became a hit, moving to Edinburgh, then London, Murphy found his calling. "I had no ambition to be an actor. But the more I did it, the more I enjoyed it," he says. "And I managed to get work."

Murphy says he would never forsake theatre. "Because I never trained, I want to learn this craft: classic texts such as The Seagull (in which he appeared this year) or The Playboy of the Western World (in which he performs next) -you don't get many film scripts that would stand next to those as pieces of writing." When Disco Pigs was transposed to the screen in 2001, however, film did become the bigger option. After appearances in The Trench and TV's The Way We Live Now, 28 Days Later merely confirmed screen work as his principal outlet.

The success of the latter, in which he was cast as one of the few humans untouched by a transmuting plague, did not surprise him. "It was a pretty good team Danny (Boyle), Alex (Garland) and Andrew (Macdonald). I wasn't aware it was a 'zombie movie' when we were making it, but a genre piece immediately attracts a huge following." Its triumph in the USA, where an alternative ending was tacked on to induce horror nuts into repeat viewings, exceeded the wildest expectations.

"For the first time, I was offered things without having to audition," he says.

"Generally, they were shite."

Big things are now on the horizon. "I've been over to Hollywood a few times, and I'll go over again before Christmas." But he expresses caution about getting sucked into the Tinseltown machinery, reeling off the usual list of mavericks that anyone worth his salt would want to work with or emulate - the Coen brothers, Paul Thomas Anderson, with extra points awarded to Sam Rockwell and Johnny Depp. Just how close to the popcorn he might be, and a measure of the regard in which the studios hold him, came in the recent invitation to screen test for the part of Batman, no less (the role eventually went to Christian Bale). Even Murphy cannot suppress the fan's enthusiasm for too long. "It was just a blast," he gushes, citing how Batman is the coolest of all the superheroes, because there are no superpowers involved. "He's just rich. I wish my 10-year-old self could have seen me in the suit, though it was f***ing hot."

A couple of releases from before the 28 Days Later success are yet due: the American civil-war epic Cold Mountain, in which he has a small part; and the film about the artist Vermeer, Girl With a Pearl Earring (which really does star Colin Firth). For this, as a butcher's boy with a brutal mullet haircut, Murphy's research was more Damien Hirst than Dutch master -"I chopped up a few f***ing pigs and went to an abattoir" -though he did share scenes with the ingenue du jour, Scarlett Johansson. Thank God, he says, that the success he is enjoying did not happen to him at her age. "She's young: she turned 18 on the picture. She's a fantastic actress. But I defy anyone who's 18 to say they are a fully rounded person."

Whatever the misplaced comparisons with Farrell, Murphy will definitely not be going down Farrell's celebrity route: Heat magazine, Britney Spears and all. But he says that Farrell has also been misrepresented, to a degree. "You know, Colin, he likes to go out, he likes to meet people, and the thing about that is it's not contrived. The thing I have a problem with is people who just do it because they think it's going to help their career. That's just the way Colin is. He's gonna get photographed because he goes out, but it's not like a strategy. I like to go out as much as the next guy, but I just prefer to do it in my local pub."

Murphy has recently been shooting Red Light Runners with Harvey Keitel and Michael Madsen, a sort of mafia/CIA/heist caper set in the UK, in which he plays "a cockney geezer". "Meeting Keitel was a bit overwhelming, actually," he confesses.

Perhaps an actor whose career he would aspire to? Certainly. Make the right choices and anything is possible, he says. "People now will hopefully be able to identify me rather than go, 'Who?', you know? But every project I do, I want to look back and say, 'I believed in that.' I don't ever want to go, 'It's awful, don't mention it.' I don't know if I'll manage that, but that's my ambition.