I meet Cillian Murphy between a matinee and evening performance of Love Song directed by John Crowley, the Sonia Friedman Production he's starring in at the New Ambassadors theatre in the West End. I tell him about a dubious legend involving Richard Harris, who reportedly got riotously smashed after a matinee, met a woman in a pub, then cajoled her to the evening performance. As the curtain rose and the first lines of dialogue were delivered, Harris apparently shot bolt upright in his seat exclaiming, "Fuck, I'm in this!" before diving for the fire exit.

If Murphy were the same hell raiser as his countryman, you would never know. He likes to keep his private life private. In the bar, he nurses a coffee in a paper cup, which he's brought in with him. Waitresses coo over him. Yes, they know him here, but he's a good-looking fella too—shockingly blue eyes and lashes you could paint a bedroom with.

In Love Song Murphy plays Beane, an obsessive, Beckett-esque character from a sparse, solitary existence who invents a whirlwind romance with a sparkly, imaginary female, played by Neve Campbell. Kristen Johnston and This is Spinal Tap co-creator and star Michael McKean play his concerned sister and brother-in-law who are ultimately transformed by Beane's positivity. It's a sweetly comic performance, and a return to Murphy's roots on the stage. Leaving behind his law degree from University College in his hometown of Cork, then passing on a record deal with London label Acid Jazz with his band Sons of Mr. Greengenes, he took instead the role of Pig in Edna (sic.) Walsh's play Disco Pigs in a touring production. The rest, as they say, is history.

"I love being back in the theatre," says Murphy, sporting a bristling beard for the stage that makes him look a little older than his 30 years. "You leave a production a better actor, because you're practicing and constantly evolving a character. The movie actors I admire have always done quality theatre, like Michael Gambon, like Gabriel Byrne or Pacino. It challenges you tremendously. Not that film doesn't, but it's a more direct and immediate thing on stage. I wanted to do comedy just to see if I could, and I think that I can. The cast are fantastic too, and it's pretty cool jamming guitar backstage with David St. Hubbins (Michael McKean)."

From starring in Ken Loach's 2006 Palme D'Or-winning film The Wind That Shakes the Barley to riding a Hollywood behemoth as the best Batman villain since Jack Nicholson's Joker, it's been some journey so far. Director Christopher Nolan auditioned Murphy for Bruce Wayne in Batman Begins. And though Christian Bale ultimately landed the role, Murphy got the part of Jonathan Crane, the psychotic psychiatrist who becomes the twisted Scarecrow. Murphy says he can't talk about whether he will be in future instalments of the franchise. I hope that means he will. The actor has also become a dad for the first time. His wife, artist Yvonne McGuinness, gave birth to their son Malachy in December 2005.

Working with Loach is something Murphy says he never tires of talking about. "I've never had an experience like it. He is a masterful filmmaker," he raves. "It's completely divorced from the normal film set. There's none of the bollocks that's involved, there's no shouting action, no marks, none of that. But he's also one of the nicest people I have ever spent time with. He's genuine and passionate. It was so untouched by money, by marketing, by cynicism of the film industry. It was a totally pure experience and that is so very rare."

"I was at home, shooting in Cork, in places I knew like the back of my hand," he continues. "It was extraordinary and I'm very, very proud of what the film says. And then there was the whole thing in Cannes. We had a ten-minute standing ovation, and that's an experience I don't think I'll ever top in this game. It doesn't pull any punches, but it didn't win on its politics. It won because it was a fucking good film."

Released in the U.S. in February, Loach's film received a mauling from the U.K.'s right-leaning press for what one newspaper called its "poisonously anti-British message." Another called it "a hard-line Marxist distortion of history." Taking on the rarely documented events of the Irish insurgency against occupying British forces in 1920 and the brutality of the notorious Black and Tan squads sent to crush the rebellion, the story resonates all the more now with the war in Iraq. That a devastating civil war broke out in Ireland once occupation ended is worryingly prophetic.

"I fucking loved the controversy," says Murphy. "The criticism from the right-wing press was so easily dealt with, and gave Ken and Paul (Laverty, the screenwriter) a real forum, and I've never met people more eloquent than those two guys. So it was brilliant to see them take [the critics] on and win so convincingly. It was only two generations ago. Everyone in Cork has some direct or indirect involvement; our family lost a cousin to the Black and Tans back then, and my grandfather was randomly shot at."

Loach may have given Murphy a Palme D'Or for his resumé, but equally pivotal to his career is director Danny Boyle. By casting Murphy in the lead role of Jim in the terrifying sleeper hit 28 Days Later…, Boyle made him leading man material.

"I was relatively inexperienced when I did 28 Days Later…, and it was undoubtedly the watershed for me, working with a proper established director," Murphy recalls. "I then went off in the interim and did a lot more films, so to come back and work with him again with more to give was really nice. He is an exceptional director and he gets exceptional performances from people. He never sits down on set, ever, and that enthusiasm is infectious. People are willing to bend over backwards for him."

Murphy's back with Boyle for Sunshine, the director's first crack at sci-fi—which would have come considerably earlier had he not passed on Alien 3 back in 1992. And those exceptional performances clearly come in the preparation. During rehearsals, Boyle made the ensemble cast (including Rose Byrne, Chris Evans, and Michelle Yeoh) live together cheek-by-jowl for a month in east London so they could bond as a team. He also enlisted them for lectures by physicists and astronauts, made them experience zero gravity at the edge of the earth's atmosphere where it merges with space, and sent them to visit a project in Switzerland where scientists are working on a particle accelerator that attempts to create a small "big bang."

The premise of the Alex Garland-penned story is both strangely obvious yet strikingly original, particularly in the well-worn world of sci-fi. Murphy plays Capa, the physicist in charge of a vast warhead on board the Icarus 2, the second space mission sent to re-ignite the dying sun and save the human race from extinction. The first disappeared without a trace. It's as gripping as every space film should be—loaded with claustrophobia, the fear of the unknown, and of course, the existential questions that surround the source of all life. It's that rare occurrence—a big budget thriller with a brain. Much like 28 Days Later…, the film's apocalyptic nature is tempered by an ultimate positivity. And with its intertextual nods to space films from 2001 to Event Horizon, it's as tense as piano wire.

"Alex (Garland's) thing is the 'big idea.' He can set up a premise fucking brilliantly, but at the same time, tick all the boxes of a blockbuster and ask the important questions. He created separate back stories for all the characters," says Murphy.

Though he's not keen to describe himself as a method actor ("the term 'method' has become pretty devalued and meaningless"), Murphy invests everything into his roles. For the part of Kitten in Neil Jordan's Breakfast on Pluto, he hung out with transvestites, studying how they carried themselves and "finding the woman inside" him.

"If you're going to play someone from that culture, you have to spend time getting to know it," says Murphy. "It's more of a respect thing than anything. That's the furthest I've ever gone for a part because it's the furthest away from me. If you mean, do I stay in character off camera? Then no. But do I take my work home with me? Yes, it comes home with you no matter what."

"The people I admire as actors, you never see in the papers, because they're not courting it, they're just living normal lives. People at a far higher level than me, too. There were pictures of my son taking while I was filming in New York, and that made me angry. I didn't ask for it, and I don't want it. I'm not saying I'm besieged by paparazzi, it was just once and it's really just not a part of my life," he continues.

"I hate it when people try to define this cult of celebrity thing, and be profound about it. It's just like global warming or something—it's just a nasty thing that has developed. I don't have many friends that are actors, and when I leave the set, I don't like to hang around in those circles. I have a close core of friends, most of which I've known from way back." When I tell him that he has five dedicated fan sites, he shrugs and asks, "Is that a lot?" It is, I say.

As for the future, there's a very big project in the offing that Murphy says he can't really talk about. In the meantime, there's an indie comedy co-starring Lucy Liu called Watching the Detectives about a film noir-obsessed video shop clerk. "I'm hugely proud of my body of work, and I don't want to let the bar drop," Murphy explains. "So if that means taking my time and choosing things carefully then that's what I'll do."