The Inbetweeners Movie (15), The Guard (15) (4/5, 4/5)

Starring: Simon Bird, James Buckley, Blake Harrison / Starring: Brendan Gleeson, Don Cheadle, Mark Strong, Pat Shortt

The movie of the sitcom is always an accident waiting to happen. I have a soft spot for the big-screen version of Rising Damp, but I'd prefer to keep it quiet. The reason for the genre's persistence might be the legendary box-office performance of On the Buses, the biggest UK cinema hit of 1971, proving that you can turn out any old guff if your sitcom already has an established fanbase. (Or maybe that was just the way things were in 1971).

So I confess to strong misgivings about the prospect of The Inbetweeners Movie, mainly because this C4 sitcom about a quartet of thwarted suburban 18-year-olds was very dear to my heart, and three series of it felt the right moment to call it a day. The producers thought differently – and let's rejoice that they did, because the movie is as funny as an episode of the show and three times as long. Funny? This shakes the house with funny. And it does so even though it takes the surest route to disaster for a sitcom aimed at the wide-screen: it sends the characters on holiday. (Hall of Everlasting Shame: Sex and the City 2).

The four friends – half-civilised nerd Will (Simon Bird), romantic masochist Simon (Joe Thomas), sexual braggart/ actual virgin Jay (James Buckley) and lanky simpleton Neil (Blake Harrison) – celebrate the end of school by jetting off to Crete in the hope of sun, sex, the usual: "like shooting clunge in a barrel," leers Jay, and if you don't know what that means, don't worry – nor does he. Having checked into the nastiest apartment block on the island, Will suggests they visit the Minoan Palace at Knossos – to the others' utter disdain. "You can see that shit anywhere," says Jay. So it's off clubbing in town, where their efforts at dancing, chatting up girls and holding their drink prove hilariously inept. They manage a very rare trick of being utterly imbecilic yet strangely loveable.

Credit for that must go to the writers, Damon Beesley and Iain Morris, whose steady rattle of coarse jokes is matched to an instinct for detail – like the fact that Neil has packed a box of Frosties in his suitcase. They are smart, too, to keep Will, the original "briefcase wanker", as a narrator, leavening the raucous bog-wall humour with ironic comments on his (and their) inadequacy. He realises, for instance, that in their holiday duds they look like "the world's shittest boy band". Inadequacy, but also an odd kind of innocence, notably in their mortifying attempts to connect with women and their belief in friendship, even if that concept might only mean saving your mate from sleeping face-down in an anthill. Of course there's eye-watering stupidity to spare – as when Neil does a huge poo in the bidet (he thought it was "a children's toilet") - but if the laugh that got from the premiere audience is anything to go by, these stupid boys should be the homegrown hit of the summer. It might even banish the memory of On the Buses.

In John Michael McDonagh's roguish black comedy The Guard Brendan Gleeson plays the funniest Irish copper since Colm Meaney's turn in John Crowley's 2003 Intermission. Gleeson is Gerry Boyle, a pudgy, unconventional (he'd laugh at "maverick") Garda sergeant in a quiet Connemara coastal town, the sort of law enforcer who tut-tuts over a car-crash of dead joyriders and then pockets their drugs for his personal consumption. He frolics with call girls, insults his colleagues and appears close to nobody but his ailing mum (Fionnula Flanagan), with whom he chats about Russian novelists.

The stakes get higher than pilfered acid tabs when an FBI agent, Wendell Everett (Don Cheadle) arrives in town to lead a major operation against a drugs-trafficking gang. At first taken aback by Boyle's provocations, Everett eventually twigs that the racist remarks and off-colour jokes may just be a screen, and that the country cop is shrewder than he cares to appear: after all, he's the first to connect the rumoured drugs shipment with a local hit disguised as an occult murder. "The significance of the pot plant has me somewhat perplexed," admits Boyle, one of the milder lines of McDonagh's scabrous, cynical, literate sceenplay, in which even the villains (Liam Cunningham, Mark Strong, David Wilmot) lace their profane talk with learned references to Nietszche. Drollery abounds. There's a fabulous moment when Boyle, doing an arms deal with a Provo (the great Pat Shortt), is surprised to learn that the IRA had gay recruits. Oh sure, replies the Provo, "back then it was the only way to infiltrate MI5". If McDonagh's name seems familiar, that's because he's the brother of Martin, whose In Bruges had a similar hard-boiled erudition, and also gave a starring role to Gleeson.

If he was good in that, he's great in this, revelling in the superb back-and-forth with Cheadle's straight-arrow Fed. The latter's disbelieving attitude towards the reprobate Garda echoes our own – is Boyle spinning a line, or was he really a competitive swimmer at the Seoul Olympics? Is his assumption of the Fed's background in "the projects" another little tease – and does he really not get the difference between dollars and pounds? Gleeson's twinkly contrariness makes it difficult to tell; we are all, indeed, "somewhat perplexed". Cheadle himself does a very good line in longsuffering deadpan, particularly in the running gag of locals asking him whether he's in the FBI's "Behavioural Science Unit" (no, he's just in criminal investigation). His gradual warming towards Boyle is nicely worked, as is the unresolved mystery of its finale. McDonagh has shaken up a cocktail of genres here – odd-couple, fish-out-of-water, thriller caper – and hit upon a pure draught of comic gold.