If you haven't seen it yet, do yourself a favour and rent ONCE. It's a lovely movie, very low-key with the feel of a documentary, though it isn't one. It could be called a musical, though it's there's nary a hint of BRIGADOON about it (although I loved that movie too, if I'm being honest.) I think ONCE is best described as an urban fairy tale. You don't even know the main characters' names; they're described simply as Guy and Girl.
The music is charming, the characters endearing, and you'll want to just sit quietly when the film is over and let its gentle influence melt into your bones.