Another old thread resurrected from the dead for no apparent reason; probably it is the only one on the subject here that doesn't contain substantive information. For more up-to-date information, very easily found through the onsite search engine, see, among a great many other discussions:

Origins: Puzzle:Edmund Fitzgerald and Bobby Sands

...where it is explained quite clearly that Bobby Sands' poem was set to the tune of The Wreck of the Edmund Fitzgerald, which Gordon Lightfoot wrote.