John Michael McDonagh sounds delighted to win the Guardian first film award. "I'm really chuffed," he says, via telephone from Australia, where he's on holiday. "It's terrific. I read over the longlist and thought there were a lot of very strong films there. Snowtown is one of my favourite films of the year, Attack the Block is probably the best British film of the year – so to come out on top, I'm pretty surprised and very pleased."

McDonagh's black comedy The Guard, which pairs Brendan Gleeson and Don Cheadle in an acerbically funny film about a wily Irish copper and a straight-arrow FBI agent taking on drug smugglers, defeated all comers, including Paddy Considine's Tyrannosaur, Richard Ayoade's Submarine, and David Michôd's Animal Kingdom – becoming, in the process, the first non-British winner of the award. (Previous winners include Unrelated, Sleep Furiously and, last year, The Arbor.)

But McDonagh is wary of being a poster-boy for Irish cinema. "I was born in Elephant & Castle, and I live in Camberwell. Because my name is Irish, and I'm identified with a Irish movie, people assume things – apart from summers in Ireland I've barely set foot outside London for 45 years. It's a strange one."

The way he tells it, his decision to relocate, cinematically, across the Irish Sea was born of practicality, if not desperation. "I never got backing from the British film industry; the backing I got was from Ireland. I've been writing screenplays since I was 24, and I've been trooping in and out of film company offices in London for 20 years – and got nowhere. These guys give you advice on how to improve your script, while they make the same crappy movies they've always made. It was frustrating.

"To be honest, most producers in Britain are liars, thieves or morons. It makes me sound bitter, but you take your encouragement from where you get it, and I got mine from Ireland."

Mentioning the "M" word – his brother, In Bruges director Martin – is less of a sore point than you might think. "The whole foundation of The Guard was because I was friendly with a lot of Irish actors through my brother's theatre work. I had a feeling that if I could get the money, it would all fall into place quickly – which it did. When you walk on to a set and you know most of the people, it's still nerve-racking, but it feels a bit easier. As it turned out, the shoot was a breeze."

McDonagh admits to being a tad cheesed off at the way his brother beat him to a feature film debut, but is happy that "the positives outweigh the negatives". "It's understandable that critics compare us, especially as we've only made one film each. It'll only be after we've made our second and third films that there'll hopefully be a clearer differentiation."

As far as that goes, McDonagh says he's planning to go "arthouse" for his next film, a clerical child abuse drama called Calvary – again starring Gleeson. "I had an unexpected success with The Guard, so now I want to go out on a limb a bit," he says. "People can smell a fake a mile off. I'm in the business of making big movies, that can be accepted anywhere. You're trying to tell a sincere story, and if that offends, so be it."