The Guard, John Michael McDonagh, 96 mins (15)

The FBI descends on the West of Ireland and runs into Brendan Gleason's bullish, wayward policeman

John Michael McDonagh's comedy thriller The Guard may be about a bad cop who turns out to be a tough one but it skews closer to Father Ted than to Dirty Harry.

Irish to the core, and deeply mischievous, it has a pugnacious anti-hero in Gerry Boyle (Brendan Gleeson), a sergeant in the Irish police force, the Gardaí. First seen popping ecstasy taken from a hapless joyrider, Boyle is a small-town copper in Connemara, on Ireland's west coast. He wears his uniform with punctilious pride, all the better to flout protocol.

When FBI man Wendell Everett (Don Cheadle) arrives to spearhead a hunt for drug smugglers, Boyle taunts the African-American officer with inflammatory barbs – "I thought only black lads were drug dealers" – then cheerfully pleads: "I'm Irish, racism's part of my culture." Everett eventually muses to Boyle: "I can't tell if you're really fucking dumb or really fucking smart." In fact, Boyle loves playing the bull-headed troublemaker to wind everyone up, and playing callous and corrupt to blind people to his tender heart and moral probity.

Boyle's character unfolds tantalisingly and unpredictably – and perhaps not 100 per cent coherently, but I'm not sure that matters, given an overall tone of surreal sitcom. He is set up as a lonely, unhappy man, seen waking bleary-eyed in a bedroom painted in a green, more bile than emerald, with a solitary Daniel O'Donnell poster on the wall. Gradually, we realise that writer-director McDonagh is toying with us: Boyle proves to be a thoughtful melancholic whose bookshelves are stacked with old orange Penguin spines, and enjoys listening to Chet Baker ballads. He's also tenderly devoted to his incurably ill mother (Fionnula Flanagan, who shares some sweet, pithy scenes with Gleeson). And he likes to relax by putting on his glad rags (fedora, bow tie and rakish scarf – something of a Keith Floyd look) and getting together with hookers in kissogram police uniforms.

The drug-smuggling plot feels a bit cursory yet it moves us towards a satisfying showdown, with all guns blazing. But there are two more pressing reasons to see the film. One is Brendan Gleeson, in a role that's broader and more cartoonish than his rueful assassin from In Bruges, by the director's brother, Martin McDonagh, (an executive producer here). In The Guard, Gleeson plays more obviously on his bullish physicality and imposingly monolithic way of filling the screen – whether Boyle is donning his uniform, looking scarily authoritarian, or striding out of a cold sea, Triton-like in a wet suit.

The other reason to see the film is a script generously packed with outrageous one-liners. There's some nice sparring between Gleeson and Cheadle, who gamely plays along as long-suffering straight man. But some of the best zingers are tossed off with casual abandon. I won't squander them here, but my favourite comes when Boyle accuses a scrawny little boy of stealing a pistol from a bag of arms he's found; the tyke looks at him in outrage and complains: "It's like the Birmingham Six all over again."

Occasionally, you feel that McDonagh is overdoing it verbally. The badinage between the three heavies (Liam Cunningham, David Wilmot and Mark Strong) can feel routinely facetious. When they start bickering about Nietzsche and Bertrand Russell, or when we're treated to a speculative digression on Bobbie Gentry's country hit "Ode to Billie Joe" – you feel that McDonagh has hit the "Tarantino" button on his laptop and let the script write itself. But McDonagh is also good on non-verbal humour: like the presentation of an IRA man as a stout, denim-clad bloke in a Stetson, driving an orange Beetle.

Photographed by Larry Smith to make the most of some melancholy landscapes and strikingly ugly interiors, the film is briskly functional in style, but looks and feels no more or less polished than it needs to. The Guard doesn't always earn the brilliance of its finest verbal flourishes, but it boasts a central character and matching performance that are worth their weight – and may show staying power too. The Guard cheered me no end, and I wouldn't be surprised if, some day soon, a sequel does too.

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