A case in point might be "Fields of Athenry." If I could do blue clickies, I'd link to earlier threads about it, but the essence of the story is that the song, which most people think was written by Pete St John actually dates back to the 1880s when it appeared as a printed steet ballad in Dublin. As best I recall, St' John's melody is the same, but he did change the wording in the chorus. His "arrangement" is his, but the song is trad.