Written and directed by John Carney, Once is a unique film: a low-budget, low-fi modern musical that is scruffy, soulful and full of beautiful tunes about battered hearts and bruised hopes. It’s the Hollywood musical of old, but unplugged and stripped of lavish sets and big numbers. Instead, it offers a minimal narrative and a lush lyricism that leaves a lump in your throat.

I can’t remember seeing a musical shot in the bumpy and grainy style of cinéma vérité before, but Carney manages to keep it real without losing the romanticism. His characters don’t suddenly burst into song or dance; they take to their instruments and sing as though music were their first language. Most of these haunting compositions (written mainly by Glen Hansard, of the Irish band the Frames) are performed on an old broken-down guitar by a lonely busker known, according to the credits, as the Guy (Hansard again). He meets the Girl (Marketa Irglova) on a street in Dublin. I usually hate films with characters who have existential names like this. But Carney and his two leads are so good at establishing the inner lives of their characters that you don’t notice that we never hear their real names.

Once is the story of what happens when the right person comes into your life at the wrong time. It treats its audience as adults and not, as many modern musicals do, as children who need to be wrapped in the comfort blanket of familiar pop hits or the clichés of romantic love. The guy is on the rebound from a relationship that ended badly in London; the girl is a Czech immigrant with a small daughter and a husband in the background. He works with his dad repairing vacuum cleaners; she gets a job as a cleaner.

Both are adrift from the thing they love most: music.

They meet one evening when he’s out busking. She approaches, listens and when he’s finished asks: “Who’d you write that for?” She asks questions like a curious child. At first he’s irritated, but slowly they discover their common love of music (she’s a talented pianist/ singer) and their love for each other. He sets out to make a demo of his songs to take to London, but, thank heaven, the film spares us the will-he/won’t-he-make-it drama. What really holds our attention is: will they or won’t they get it together?

The film is beautifully understated, and often funny without trying hard to make you laugh, as in the moment when you see the pair walking together down a street, her broken vacuum cleaner trailing by her side as if it were a dog being taken for a walk. Other scenes are moving and unforgettable, as when they go into a music shop and start to sing a duet together. I could have done with a few more of the Girl’s compositions, so we could learn more about her feelings, but it’s refreshing to hear a musical done in a contemporary singer-songwriter style and not the usual generic pop format. Carney has wisely used the songs to tell the emotional side of the story, so the interaction between the couple has that kind of Brief Encounter restraint that tugs at your heart.

The two leads are unlikely types for a romantic musical. Carney had originally chosen the more conventionally handsome Cillian Murphy for the Guy, which would have been a mistake. Hansard is not handsome, or handsome-but-“interesting”, or even ugly-sexy. He’s just a tall ginger bloke with a beard who looks like an ageing cherub with a midlife crisis. In other words, he’s perfect for the part. You get the sense of his terrible loneliness, as well as his hopeless love for this young woman. Irglova, making her screen debut, is not your typical beauty either, but she captures your heart by the power of her charm. Together they make one of the screen’s most appealing couples. And life imitates art once again: the two actors are now together off screen.

15, 86 mins

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010