When anyone puts new words to a tune he/she will be referred to as the writer of that song. At least this is what happens in Ireland. So Dominic Behan, say, is known as the writer of "The Rifles of the IRA" and "The Patriot Game" and many more. Come to think of it, we talk about Dylan as the author of "With God On Our Side". And the vast majority of Woody Guthrie songs have traditional melodies, though the way he could adapt a melody like "Pretty Polly" into "Pastures of Plenty" would be worth an essay in itself.   Not a problem when the melody is a traditional one but in these days when people are much concerned with their "intellectual property" it's a bit more problematic. And as McGrath of Harlow points out it can be hard to say when a tune is new and not a variation of an original melody. Consider the endless discussions about the origins of "Wild Mountain Thyme".

The link given in the 7th posting in this thread will take you to an exhaustive, and exhausting, discussion about this song. There were about six months in the '76 when Bobby Sands was not in prison. It is possible that he came across "The Wreck of the Edmund Fitzgerald" during that brief period. The conditions that he lived in the H-blocks would not have been conducive to learning new material. The strongest probability seems to be that Christy Moore got Bobby's words and used Gordon Lightfoot's melody.

At the end of the day most of us can agree that we are left with an excellent song.