Still, with a role as a cocky entrepreneur in Lena Dunham's hip TV show , not to mention that turn in - in which he played the boozy manager of an all-female Australian Aboriginal singing troupe - O'Dowd confesses it's been "a big couple of years" for him. He dubs it his "moving day", a golfing term used on the penultimate round of a tournament "where … it's time to show your wares" and move ahead of the pack.

At 1.9 metres tall, with a beard and curly black hair, he's not what you'd call conventionally handsome (though try telling that to his wife, author and TV presenter Dawn Porter, who met him at her 30th birthday party and instantly was smitten with his "big smiling face"). He and Porter still live in the less-than-glam Bermondsey in south London. And when they got married, their starriest guest was O'Dowd's cohort from , Richard Ayoade.

Yet Hollywood seems to have fallen for his insouciant charm. His latest effort is yet another string to the bow, voicing his first big animation project. Suitably titled , this 3-D offering from the creators of is set around a magical forest defended by tiny soldiers called Leafmen. O'Dowd is Grub, an uptight snail who is part of the security detail for the queen of the forest (Beyoncé Knowles). His partner is Mub, a more laid-back slug (voiced by 's Aziz Ansari).

While every cartoon needs comic relief, it's very much the job of these wise-cracking molluscs to provide the levity in a story that boasts a strong -like eco-message. "We're like a double-act," says O'Dowd. "I'm the curmudgeon [of the two]. I'm more the grumpy snail but with delusions of grandeur. So if it's Jack Lemmon and Walter Matthau, I'm Jack Lemmon."

It's not O'Dowd's first voiceover experience - he's already been on Seth MacFarlane's show and played Dr Cockroach on , the TV spin-off from the hit movie. Still, "it's a different process and it's interesting", he admits. "It's very much trying to have as much fun inside a tiny little box as you can."

As is common with animation, O'Dowd recorded much of his dialogue alone - but he did get some time with Ansari. "I did the first section with him, so we could sense what the chemistry of the characters would be." And it shows: their twosome is the highlight of the film.

Just don't ask O'Dowd how he found his inner snail (or at least if you do, get ready for a yucky reply). "I know a lot of these actors do different dietary regimes to find their characters, whether it's Christian Bale or Adrien Brody," he says. "I underwent a supplementary thing so I could extract mucus from my arse. I feel like it's really worked."

This, it should be said, is fairly typical of O'Dowd's irreverent sense of humour, which comes courtesy of a small-town Irish upbringing, with O'Dowd the youngest of five. His father, Sean, was a signwriter who played guitar and sang in pubs of an evening, while mother Denise went from parenthood to becoming a psychotherapist. His family were an "arty" lot, he says, but living in Boyle, County Roscommon, meant that your career options were either working in a call centre or the local fish factory.

Enrolling in University College Dublin to study politics, O'Dowd became enamoured of acting - and left before he finished his degree to attend the London Academy of Music and Dramatic Art. Early gigs included a small role in Mike Leigh's prize-winning and playing a stand-up comic in the black comedy , but it was that established his credentials. "More than anything it changed my life," he says.

While his co-star Ayoade has gone on to write and direct the critically acclaimed they're both keen to get back for one final crack at . "I can't imagine we'll do another series but I'd be surprised if we didn't do a special," O'Dowd hints. "Maybe sooner than you think."

In the meantime, O'Dowd has more than enough going on - what with playing a violent butcher in John Michael McDonagh's . Then there's , a comedy in which he's a rival salsa dancer to Nick Frost.

He'll also be seen in the upcoming Marvel Comics sequel - playing a suitor to Natalie Portman's scientist. "I have to say when I was there, it felt just like a normal film," he says. "There was no lightning hitting the table or anything. But it did feel like an enormous production, which is exciting. Even now, those big production pieces are quite fun, because it all feels so silly." Did he get to touch Thor's mighty hammer? "Not on camera," he says.

At least O'Dowd has the good grace not to take the business too seriously - whether it's playing a grumpy snail or hanging out with Clint Eastwood.

"I'm just glad I'm being employed in some capacity," he says. Still, for the actor who compares what he does to "moving day" in pro golf, you might say he's heading towards the top of the leaderboard right now. "Yeah," he grins. "I've made a birdie!"

opens on Thursday