The movie’s a rude, scruffy winner, a music bio/cash-in reconfigured as a deadly serious prank. It bears stressing that Kneecap is a genuine act, with rappers Naoise “Móglái Bap” Ó Cairealláin, Liam “Mo Chara” Óg Ó Hannaidh and JJ “DJ Próvái” Ó Dochartaigh all playing cheekily fictionalized versions of themselves. The group rose to local fame in 2017 and 2018 with defiant shows and mixtapes that included Irish lyrics extolling the use of recreational drugs and the necessity of standing up to the British authorities and the “peelers” (police). Seven years on, Mo Chara and Móglái Bap are no longer the scrawny teenagers they once were, but their performing charisma carries straight to the screen — you’re forgiven if you mistake them for professional actors, Mo Chara especially.

Speaking of professional actors, here’s Michael Fassbender as Móglái Bap’s father, Arlo, a firebrand republican who’s been living underground for years, supposed dead by the police and most of western Belfast while being resented for his abandonment by his long-suffering wife (Simone Kirby) and son, both of whom know his whereabouts. One of the strengths of “Kneecap” is that its heroes have lost faith in all the putative grown-ups in this battle.

Things get rolling when JJ, a Gaelic instructor at the local high school, is called in by the police after Mo Chara is arrested for spray-painting Irish slogans and refuses to speak English during interrogation. A glimpse at the lyrics in the boy’s notebook reawakens the older man’s dormant music dreams and prompts him to dust off the recording setup in his garage and invite reluctant rappers Mo Chara and Móglái Bap in. A single is released, is suppressed and quickly goes viral, blaring out of the ear buds of the kids in JJ’s classroom.

Next up is performing live at a pub full of old sods and one barmaid with an Instagram account; terrified of jeopardizing his day job, JJ hits the turntables with a tricolor balaclava hiding his face and finds his inner Beastie Boy/Borstal Boy unleashed. Which would be bad news if his girlfriend (Fionnuala Flaherty), an activist fighting to have Irish officially recognized by the British government, ever found out.

Toss in a comically inept paramilitary group called Radical Republicans Against Drugs, a fearsome police lieutenant (Josie Walker) who wants to drag Arlo in from the cold and a saucy love interest (Jessica Reynolds, delightful) for Mo Chara, and “Kneecap” would seem to have set far too many plates spinning. Yet director Rich Peppiatt, making his feature debut with a script he co-wrote with the three Kneecaps, keeps everything fizzing with visual brio and nonstop energy, balancing a youthful celebration of hedonism and altered states with a banked fury at a government that sees a people’s Indigenous language as a force for subversion.

I wish I could quote some of the group’s lyrics to you, but A) they’re mostly in Irish, B) they’re mostly unprintable even when they’re in English, and C) they’re often proudly immature in the grand hip-hop tradition, even if that immaturity has a focus and ferocity born of a very specific historical oppression. The boys can be profanely articulate in the film’s dialogue (and, off-screen, in interviews), but in their fight for their right to party, they put equal emphasis on all three nouns.

The plottier aspects of the story get resolved in too-tidy fashion — it’s a movie, after all. But the musical numbers are propulsive and hook-laden, lingering in your head and giving a defiant bounce to your step long after the credits have rolled. What really puts “Kneecap” over is that Móglái Bap and Mo Chara and DJ Próvái radically insist on squeezing what joy they can from a joyless life while keeping their eye on the long game. Somehow, they’ve come up with “A Hard Day’s Night” for the Northern Irish resistance.

R. At area theaters. Contains pervasive drug content and language, sexual content/nudity, and some violence. 105 minutes.

Ty Burr is the author of the movie recommendation newsletter Ty Burr’s Watch List at tyburrswatchlist.com.