The Decemberists' five-part epic EP The Tain is based on Táin Bó Cúailnge, an Irish legendary tale involving, among others, a young hero named Cu Chulainn, who is also the titular character in a lyric-less song called "The Blood of Cu Chulainn." What movie about Irish-Americans used this song over its opening credits? One of its stars later went on to greater fame on an AMC television show.
The Boondock Saints