John Carney‘s Once, the most unassuming and wholesomely affecting love story in years that turned into the Big Find at Sundance ’07, opens on May 18th — a little less than four weeks off. Fox Searchlight, which acquired it last February, has launched its own Once website. (The Irish version has a little more pizazz.) Here, in any event, is a fairly decent trailer that catches the mood and tone of the feature.

Glen Hansard, Marketa Irglova in John Carney’s Once

This little Dublin-shot film is about a couple of gifted but struggling musicians — a scruffy, red-bearded troubadour (Glen Hansard, best known for his Irish group The Frames) and a young Czech immigrant mom (pianist and singer Marketa Irglova) — falling for each other by learning, singing and playing each other’s songs. That’s it…the all of it. And it’s more than enough.

Calling Once a “musical” doesn’t quite get it because it’s really its own bird — it’s a tweaking (almost a reinvention) of the form in the vein of Cabaret, A Hard Day’s Night and Dancer in the Dark. On top of which it’s gently soothing in a low-budget, unforced way.. It’s about struggle and want and uncertainty, but with a kind of easy Dublin glide-along attitude that makes it all go down easy.

Once is about spirit, songs and smiles, lots of guitar strumming, a sprinkling of hurt and sadness and disappointment and — this is atypical — no sex, and not even a glorious, Claude Lelouch-style kiss-and-hug at the finale. But it works at the end — it feels whole, together, self-levitated.

Trust me — there isn’t a woman or a soulful guy out there who won’t respond to Once if they can be persuaded to just watch it. The trick, obviously, is to make that happen, and I admit there may be some resistance. Initially. But once people sit back and let it in (and they’d have to be made of second-rate styrofoam for that not to happen), the game will be more or less won. Settled, I mean.

Carney, Hansard and Irglova are starting a 15-market p.r. tour from April 30 to May 18. They’re in Manhattan on May 1st and Los Angeles on May 15th. In each city Fox will be holding special promotional showings followed by a performance and q & a in each market, which is the template that worked so well at each one of the Sundance screenings last January.