Brendan Gleeson is brilliant as The Guard.

In his uniquely Irish take on the buddy-cop format, John Michael McDonagh unapologetically hangs all the dirty linen out to dry.

Along with the typical traits associated with the dark side of the Emerald Isle, such as drunkenness, racism, corruption and parochialism – drug abuse and whoring are also thrown into a shamelessly hedonistic mix.

However unpalatable, they are all legitimate subjects for an Irish writer to tackle in a black comedy set in Ireland. Indeed McDonagh pulls no punches, writing only one fully sympathetic Irish character into the plot – a gay rookie guard, involved in a sham marriage to a glamorous Croatian woman.

Perhaps fittingly, seeing as it is set in the Wild West of Ireland, The Guard has the feel of a Western, underlined by McDonagh’s choice of a Mexican trumpet-and-guitar score by Calexico. Along with the vibrant colours of many of the interiors, this sets the scene nicely for the displays of florid rhetoric that are usually obscene and occasionally hilarious.

After a blistering opening, the story slows for a few scenes as we get introduced to the characters against the rugged backdrop of a damp and foggy West of Ireland. Although Connemara looks well on the big screen, cinematographer Larry Smith is not out to shoot a tourist brochure. Personalities take preced­ence over landscape.

By the time Don Cheadle’s character is introduced as a highbrow FBI agent named Everett, who is forced into cooperation with Brendan Gleeson’s distinctly lowbrow Gerry Boyle, the story has found an enjoyable pace that it maintains until its explosive denouement.

Gleeson puts in one of his finest ever performances as the whoring, drinking, defiantly iconoclastic Garda Boyle. It’s a perfect fit, as he effortlessly personifies the character, filling his uniform to its seams with his corpulent frame and delivering McDonagh’s script with great comedic ease.

While Garda Boyle’s mother is dying, he hires prostitutes to dress up in uniform. His response to violence is a deadpan shrug, and he uses the drugs that he finds in the pockets of crash victims. He is tactless, politically incorrect and seemingly apathetic.

However, when a rare murder occurs on his patch, we find out he is not totally irredeemable. It is his peculiarly Irish brand of defiance, rather than any crisis of conscience, that seems to drive him to take a stand against the corruption of his colleagues and trace the thread to its violent conclusion.

What really marks The Guard is McDonagh’s uncompromising script, and Gleeson’s total mastery of it. At one stage, Everett has barely begun his briefing of local authorities when Boyle interrupts him with an outrageous racial insult before explaining dryly: “I’m Irish. Racism is part of my culture.”

Although such lines made this reviewer squirm momentarily, it is to McDonagh’s credit that although he treads the line towards being offensive, he doesn’t cross it. The lurid themes and foul language never seem gratuitous. Instead, they serve to give the dialogue a realism an immediacy that drives the film.

Like his brother Martin McDonagh’s highly acclaimed In Bruges, John Michael McDonagh’s directorial debut is an accomplished black comedy with a refreshing script and strong lead actors to deliver it. Although it paints quite an unflattering portrait of Ireland, The Guard is at times brilliant and is most definitely a welcome addition to the country’s cinematic CV.