Sometimes there is a fine line between genius and stupidity; that is the idea at the heart of “The Guard,” a new film, written and directed by John Michael McDonagh.

Set in the west of Ireland, F.B.I. agent Wendell Everett (Don Cheadle) must pair up with eccentric police sergeant Gerry Boyle (Brendan Gleeson) to break a drug-smuggling ring. Wendell is the straight man, by-the-book and rule-abiding. Gerry, on the other hand, is unorthodox and inappropriate and delights in getting a rise out of Wendell at every opportunity. Wendell cannot figure out whether Gerry is an idiot or some kind of genius. The film works best when these two characters are on screen, with the the two bickering and trying to outwit the other. The chemistry between them is authentic. In fact, the film seems to be more focused on the characters than it is on the plot.

McDonagh is not interested in the mechanics of the drug conspiracy or the intricacies of police work. He is, however, interested in what the drug dealers discuss and how they live when they are not dealing drugs. This trio of drug dealers happens to be interested in everything from John Locke to the very meaning of being a drug dealer.

The leader of the gang is played by Mark Strong, who has become the go-to actor for movie villainy. He has also played feared gangster Frank D’Amico in “Kick-Ass” and Lord Blackwood in “Sherlock Holmes.” Here Strong portrays Clive Cornell, a British national who hates dealing with Irish cops and Irish criminals. Though he is menacing and clearly dangerous, he isn’t a mindless thug. Cornell is existential about his life as a drug dealer and wonders if he will ever encounter a worthy opponent.

McDonagh is at his most confident when he is dealing with characters and dialogue. He sews together threads of profanity so well  it sounds like poetry. When the film does lose its footing, it is due to the plot. At times, the storyline feels rushed or simply lacks needed detail. However the first and final acts are so well constructed that any shortcomings in the middle — and there aren’t many — are forgotten by the time the credits roll.

Though at times McDonagh’s writing leaves something to be desired, the visual aspects of the film do not disappoint. The cinematography is sometimes reminiscent of a western film, with its lone figure standing against an expansive landscape. Many films originating from or set in Ireland portray the Emerald Isle as dreary and gray. This is not the case here – “The Guard” is lush and colorful, with vivid greens, blues and reds dominating the screen.

It is not set in a storybook Ireland with twinkle-eyed drunks and plentiful sheep. Gerry makes it clear that Galway, the setting of the film, is far superior to Dublin. There is a definite sense of place in the film; this is not just Ireland, this is the west of Ireland. While a lot of that might go over the heads of an American audience, it certainly does not detract from the film.

While the dialogue, acting and overall cinematography are outstanding, truly the film’s greatest strength is Brendan Gleeson. Gleeson is one of “those guys” – a character actor who appears in at least three films every year and makes each one better. Here, he plays the title character and you will not ever forget it. Gleeson is a big man, playing, Gerry, a larger-than-life figure who is so grand that the tiny isle of Ireland cannot contain him. The film rests on his broad shoulders and he carries it the whole way.

Danny Marchant can be reached at [email protected].