John Michael McDonagh: Stand aside, junior – big brother coming through

'The Guard' sounds like indie hit 'In Bruges' and the directors share the same surname. But this time it's McDonagh major in charge.

Dressed in a sharp pinstriped suit, his head closely shaven, John Michael McDonagh cuts a striking figure. But it's when he opens his mouth that I'm taken aback, not least because I've just watched his directorial debut, The Guard. Like his younger brother Martin, who wrote and directed the 2008 hit-men tale In Bruges, he's delivered an acid-sharp comedy starring Brendan Gleeson. With the humour as black as a pint of Guinness and just as Irish, it's something of a shock to discover that McDonagh sounds as south London as Arthur Daley.

He grew up in Camberwell, and calls himself London Irish. "I don't feel particularly British. I don't feel particularly Irish," he shrugs. "So I guess London Irish is the best way to describe it." Still, that's not to say McDonagh is an interloper. Growing up, he spent his summers in his mother's home, County Sligo. And 18 years ago, when his parents moved back to Galway, where The Guard is set, it afforded McDonagh the chance to regularly fly back and soak up the ambience.

All perfect research for The Guard, which casts Gleeson as Sergeant Gerry Boyle, a "small-town cop" who couldn't give a fig about the job. "I feel like I know the rhythms of speech," says McDonagh, 43. "I know the way they speak, the way they say things. I find that south-London people have a similar abrasive, sarcastic view of life. They don't like people who are too egotistical. They cut them down. Cutting people down gets a lot of bad press, but I think it's quite a good leveller. Some people deserve to be cut down."

In the case of The Guard, Boyle is like the unruly pupil at the back of the class who is far smarter than he's given credit for. "Jaded and bored by everything", he'll drop acid, hire call-girls and deliver a torrent of politically incorrect one-liners, making casually racist remarks to the hard-ass FBI agent (Don Cheadle) who arrives from the United States on a drug-smuggling case. "He's essentially not corrupt or racist," argues McDonagh. "He just knows everyone around him is. He's an agent provocateur – he's trying to provoke reactions so people reveal their true personalities."

When we meet, it's just before the Irish release, and McDonagh is understandably nervous about how audiences will react. "They're very insecure about the way they're perceived by the rest of the world. By America especially and the Brits," he says. "And if you're basically suggesting there is a racism at the heart of Irish culture, then I don't know how that will go down. We'll see." In the end, he needn't have worried. Knocking Bridesmaids off the top spot when it opened, the film has since taken over ¤2m (£1.75m) and was awarded special mention in the first feature category at the Berlin Film Festival.

Doubtless this will secretly delight him. With a hint of sibling rivalry, he points out that while his brother's film In Bruges played well in Ireland, it "didn't do as well in the States". Indeed, the film only took $7.8m in the US – McDonagh blames the marketing – though its worldwide tally was $33m, healthy enough for a low-budget effort. "It's one of those films that's got its reputation since," he concedes. "When it came out, the reviews were a bit mixed. There was a slight resentment towards it."

You have to wonder if it was a sentiment McDonagh was feeling; after all, he'd been trying for years to get a screenplay off the ground while his brother made his debut off the back of a series of highly acclaimed Tony-nominated plays (including The Pillowman and The Lonesome West). To his credit, McDonagh doesn't bat away the question. "Well, he was a very successful playwright, and I hate the theatre. So I never cared about his success with plays. But when he got his film out first, I was resentful and jealous. But then I got over it" – he flashes a quick smile, obviously revelling in the fact that he now has the upper hand – "and I got this one out."

Could they ever work together? He gives it some thought, even citing a potential project he's written called "The Bono Gang", a 1910 "Brian De Palma-type period epic" about French criminals. "But he probably wouldn't co-direct a script of mine. And we'd probably have too many arguments anyway, so it's probably not a good idea." He says they could never sit together writing. "He came round to watch football a few weeks ago, and he finished off the last of my cheese in the fridge. And I had a massive row with him. So if we're arguing about that, we're probably not going to work together on a film!"

He grudgingly admits that he and his brother share a certain outlook on life. "We lived together for a long time, watched the same movies, read the same books, listened to the same music. So you're going to end up with quite similar sensibility." With his father a construction worker and his mother a part-time housekeeper, McDonagh left school when he was 16. "I was on the dole and I'd just sit in all day watching Barbara Stanwyck movies on Channel 4," he recalls, and we lament for a moment the days gone by when television broadcasters used to run seasons of films.

He started out by writing five "really bad novels". None of them got published. "I was up in bed, and I could hear the plop of the manuscript, as it was returned. It gets to that dispiriting level where you think, 'I'm going to have to get a proper job.'" The thought of getting up at 7.30am for an office job he hated propelled him to keep going. Turning his hand to screenplays, he "got an immediate response" to his work. "I felt I had a facility for it." So much so, that when he wrote The Guard, he finished it after only one draft, almost unheard of in screenwriting.

Even so, it took until 2003 to see his first script produced, with the release of Gregor Jordan's Ned Kelly, which starred the late Heath Ledger as the infamous Australian outlaw. It was McDonagh who recommended the source book, Robert Drewe's Our Sunshine, to the producers to option. Asked to adapt it, he was less than happy with the resulting film, though. "It didn't go the way I'd hoped. I didn't get along with the director. He's a pretty humourless man."

Now he has no need to write scripts for others. Aside from "The Bono Gang", he has a big-budget screenplay called "Chaos Inc", about a Buddhist private eye in Las Vegas, in the works. First, he wants to do another film with Gleeson, about a priest whose community turns on him. "If you write about a priest in Ireland, you're into Father Ted territory," he says, "especially if it's got elements of humour in it. I want to get away from that." Like his brother, he seems determined to leave his mark on Irish culture.

'The Guard' opens on 19 August