Running time: 88 minutes.. .

JOHN Carney, one of the creators of Bachelors Walk, the great TV drama of Celtic Tiger malaise, brings to the big screen a musical of sorts set in modern-day Dublin. It stars Glen Hansard, the Frames frontman of scruffy beard, puppy-dog eyes and chippy temperament, and Marketa Irglova, a Czech piano player and singer who has a hopeful sparkle of a smile. Theirs is a story of arms' length romance. He's a lonely hoover repairman called Guy, who lives with his elderly dad (Bill Hodnett) and busks his own beautiful songs on Dublin's Grafton Street; she's a Czech immigrant with a broken hoover who can't afford a piano, but duets with him in music shops.

She adds harmony to his plaintive, folksy songs and encourages him to record his music. And that's about it.

The film won the World Film Audience Award at Sundance, and so comes to these shores with the expectation of a movie where the oft-heard, excusatory phrase "not bad for an Irish film" need not apply. But I felt deflated by the experience. It is shot with a ragged, documentary realism and has the cosy quality of a film made on a tiny budget. It has moments that are heart-warming, moving and charming, but only when Glen Hansard lets loose with song . . . and this is its problem. Hansard has a voice that wraps around you. His voice reaches out of the film and connects in such a way, it is like an amnesiac wash: when the songs fade, it's difficult not to see a film bereft of any real story, a threadbare framework stretched to breaking point. It looks more like an 88-minute music video with snippets of improvised conversation, than a musical drama . . . a project that started out as something else but got talked up into feature film.

The romance, which has echoes of Sophia Coppola's Lost in Translation, begins on Grafton Street. Hansard's Guy has an acoustic guitar with a worn-out hole in its side, and his heart too is hollowed out by the loss of a long-time girlfriend. Irglova's character (nominally titled Girl) is drawn to Guy, his mournful songs reminding her of what she has left behind . . . a husband she is now unsure about and her beloved music. So they strike up a partnership . . . he fixes her hoover, and she vacuums up his negative energy, encouraging him to form a band and get a loan to record a demo. They are awkward with each other in the way soon-to-be lovers are, feeling out the territory before daring to advance. But there is something that holds them back, and their spark ignites only in music.

She lives with her mother and her son in a flat on Mountjoy Square. It is frugal, like an old student bedsit . . . a long way from plush Celtic Tiger Ireland. Her neighbours, young eastern Europeans, take up their couch each evening to watch Fair City and learn English. It makes for a funny, genuine moment: "Are you not pregnant?"; "What's the story?" they mimic, in bad Dublinese, and it looks like Once is going to shine a light on the modern immigrant experience. But instead, it takes a subtle, odd turn, becoming a rear-view mirror of Ireland past: Guy plans to escape his hardscrabble Dublin life by going to London, his dreams built around making it in the music business with his new demo and winning his girlfriend back.

And you could be forgiven for thinking it was made 15 years ago, when Carney himself was a bass player in the Frames. The maudlin atmosphere, the raggletaggle buskers who join Guy's band, and the surly studio engineer with the leather jacket look like a cultural Kodak moment of Dublin in 1992, a time when London was still a better option. It's awash with nostalgia that's made more confusing by the verite realism of the camera, which plonks the film right in 2006 when it was made.

You could argue that Once is a film where nothing happens, and that slightness is its very point.

But Hansard and Irglova are amateur actors, and they do not have the chops to make the insignificant moments shimmer.

Their strength is in a musical expression that dwarfs their acting. In song they can unleash real emotional power. It's a small film propped up by the big warmth of their song.