Written and directed by John Carney, the award-winning 2006 musical film "Once" — a simple, bittersweet love story set in his hometown of Dublin and featuring wonderful music by co-stars Glen Hansard and Marketa Irglova — was perfect on its own. Wouldn't a Broadway stage treatment ruin the quiet, refreshingly unsentimental tone that made the low-budget indie based on a book of the same name by Edna Walsh so beloved?