It's kind of funny, then, that at the precise moment his star was on the ascent, the 34-year-old actor chose to pack his bags and head home to the tiny town of Boyle, County Roscommon, Ireland, his boyhood home.

As hysteria raged across the pond, O'Dowd quietly began working on Moone Boy, his fabulous, self-penned, autobiographical comedy about his childhood in 1980s Ireland.

"It's weird, people often say to me, 'Wasn't it kind of counter-intuitive to go back?' but the thing that made sense to me [at the time] was to go away and do something different," he explains.

"Actually, that's also why I went away and did The Sapphires.

"I wrote a lot of Moone Boy while in a hotel room in Albury.''

The sitcom stars terrific young newcomer David Rawle as Martin Moone, the 12-year-old version of O'Dowd. Chris stars as Martin's imaginary friend, Sean Murphy. The half-hour show, shot in his hometown, is at once hilarious and touchingly real, his character a far cry from the roles he was shooting in the US and Australia at the time.

"I wrote a lot of the first series on the plane," O'Dowd explains. "I was shooting something in LA at the time and I would come back every week and shoot some more in Albury, so I wrote a lot of it travelling around with The Sapphires."

That is not so hard to believe - after all, there's not exactly a plethora of options for things to do after dark in rural NSW?

"Ah, well, we found some pretty good pubs out there!" O'Dowd laughs, before adding that he relished the chance to work on the script while filming in Australia.

O'Dowd says going home to Boyle soon after, and getting back inside his 12-year-old head, was a lovely distraction from his increasingly frenetic Hollywood showbiz life.

"I have always found it pretty easy to recall stuff that happened around that time [in my life]," he says.

"I looked into it a little bit and apparently between the ages of nine and 13 we take on things deep into our memory. It's a very spongy time.

"And it's before I started drinking, so I suppose there's that too.

''Our show [in its third season] is going to get into trouble when Martin turns, like, 15, because I can't remember a f---ing thing after that!"

But, as O'Dowd explains, the memories came rushing back as soon as he found himself at home again in his old childhood stomping ground.

"[We filmed] at my Mum's, in the house I grew up in - it was very surreal," he explains, "when you're used to big film sets. But also, it felt very normal.

"Everybody was very supportive. It was the first thing I had written, so it was nice to be around people who only want the best for you."

So did he ever bump into familiar faces while there?

"Well, I mean - everybody!" he laughs. "It's a properly small town, so there would be nobody there I wouldn't know. It was very odd. People would turn up on set with boxes of chocolates for the crew. Weird. But really sweet.

"Now, on the other side of it, because the show has become so successful [in Britain and Ireland], there's loads of signs directing people to 'Boyle: Home of Moone Boy' on the motorway. It's bizarre!"

And big posters of O'Dowd's head to go with it?

"Yes! I look like a freak," he laughs. "I have never been fond of big pictures of me. And I'm familiar with how fame can ebb and flow. As soon as things go badly, people will be drawing dicks on my head! I can see it! Actually, I'm sure it's happened already."

So what does O'Dowd think his 12-year-old self would make of all the attention? Would he have been out there, texta in hand, providing billboard adornment?

"I don't think I would have been so brave," he says. "I was more 'keep your head down and don't get into trouble. And sooner or later you'll convince a girl to kiss you'."

Moone Boy, UKTV, Thursday, 10.05pm