I have only heard Twa Corbies by Paul McNeill. Anyone know what that melody is?

Look round other threads on this song. I've never heard of Paul McNeill, but most likely he was singing the Breton ballad tune "An Alarh" ("The Swan"), which as Jim MacLean says was attached to the Scottish ballad by Morris Blythman (pen name "Thurso Berwick") in the 1950s.