The film is based on the book of the same name by Roddy Doyle. This was Doyle's first novel, and part of his Barrytown Trilogy, centring on the Rabitte family. The second and third books, The Snapper and The Van were both made into films by Stephen Frears, although the name of the family was changed to "Curley" to avoid the perception that they were sequels to The Commitments. Although they were. In a way.
Another note is that the quote mentioned above was changed for the film version, as in the book the "n" word is used rather than "black".