Once a musician, always a musician — at least as far as spark, if not skill, is concerned. That sincere belief in music’s lasting creative and cathartic powers propels Once — a sweet, simple, stunning film about an Irish guitarist, a Czech pianist and the music they make together.

Whether Once connects as the greatest unconventional musical since Bjork’s Dancer in the Dark depends in part, as did Dancer, on what you think of the tunes themselves. Actors / singers Glen Hansard (of the Frames) and Marketa Irglova’s flowing acoustic work resembles Damien Rice and former cohort Lisa Hannigan, only with less lyrical gloom and more backing-band beats.

The smart money says you’ll set aside cash for Once’s soundtrack to complete the experience. The CD can’t recapture the film’s raw, vibrant performances, boldly presented in full, not snippets, by former Frames bassist and screenwriter-director John Carney. But it does offer an outlet for the songs’ memorable melodies and the film’s achingly realistic emotions long after the lights come up.

Hansard and Irglova’s character names are Guy and Girl, and these-actors-singers bring all the identifiable peaks and valleys of people with conflicted souls to these identity-free roles.

When not a “Hoover fixer sucker guy,” Guy is a Dublin street busker playing what people want to hear by day. Only late at night does he play painful, passionate originals to unleash his anguish and anger over a breakup with his unfaithful girlfriend.

One of Guy’s own songs inspires Girl, a separated single mom who sells magazines and roses, to toss a change and encouragement his way. He fixes her busted vacuum, and she takes him to a music shop where she’s allowed to play classical piano pieces in the back during the lunch hour.

His haunted eyes, her optimistic smile and their complementary musical talents lead to a riveting, uninterrupted take in which they piece together a ballad called “Falling Slowly.” Verse by verse, the song reveals two people speaking the same emotional language and, even if neither yet knows it, sharing a dream of composition. (In one great scene later, Irglova raids remotes, clocks and toys for batteries to keep a CD going.)

Eventually, they scrape together a band and a bankroll for studio recordings. And Once would seem to be pointing down a path to romantic harmony a la the entertaining but easy Music and Lyrics. Here, a courtship is complicated not by plot points, but by how emotional torment and musical creativity often can go hand in hand.

Irglova’s own emotionally exhausting song, showcased late in the film, echoes back to her early comment that no one writes music like that about people they no longer care for. Once then becomes a bittersweet ode to how late-night plans of wild escapes and different lives are a dream ended when morning light exposes them as mere ideas.

Like all great pop songs, you can’t wait to hear how it ends, and few conclusions feel as complete as that in Once. For a film so joyfully attuned to the tumultuous nature of composition, it’s no surprise that this is one of 2007’s best.