Whoops! I spoke prematurely. One of the links in this thread actually goes to a forum where Gordon Lightfoot took questions from the audience, and in the course of that thread he says concerning the "Wreck of the Edmund Fitzgerald":

"Topical songs, you know... are very difficult to come by. Every once in a while. And the Edmund Fitzgerald really seemed to go unnoticed at that time, anything I'd seen in the newspapers or magazines were very short, brief articles, and I felt I would like to expand upon the story of the sinking of the ship itself. And it was quite an undertaking to do that, I went and bought all of the old newspapers, got everything in chronological order, and went ahead and did it because i already had a melody in my mind and it was from an old Irish dirge that I heard when I was about 3 and a half years old. I think it was one of the first pieces of music that registered to me as being a piece of music. That's where the melody comes from, from an old Irish folk song."

The thread in which he says this is on this page.