Hi Kim! The nature of folk song is that just about everybody gets the song from someone (or someplace) else (sometimes even the original author, after all, every son has a father). I doubt my great grandfather was an exception vis-a-vis "Skibbereen."The phrase "a blight came on the land, and the sheep and cattle died" coupled with "well I do remember the year of forty-eight," should establish that the song, almost surely, speaks of The Great Famine of 1847-1853 and the Young Irelander Rebellion of 1848. On a (perhaps unnecessarily) personal note, my great grandfather was probably born about that time (circa 1850) because my father was the youngest of 5 children and was born in 1901.
Martin Ryan's advice above that "Skibbereen" is listed in a book from 1901 is very helpful. (Thanks, Martin!) As I said earlier, I'm going to keep an eye open as I look through 19th century American songsters to see what is the earliest date I can find. Some New York concert saloon songsters, for example, are rather obscure in that only a few copies have survived, maybe it'll pop up in one of them.
Regarding the melody, I've never heard "Skibbereen" sung to more than one melody. I knew Joe Heaney fairly well 20-25 years ago and I recall my greatgrandfather's melody was very similar to Joe's. The forthcoming Folk-Legacy CD, "Irish in America: A Musical Record of the Irish People in the United States 1780 - 1980" includes a simply incredible song entitled "Scovill's Rolling Mill." The melody I used when I recorded it was my greatgrandfather's "Skibbereen."
All the best,
Dan Milner