I love sad songs too!According to the Ossian Publication "Folksongs & Ballads Popular in Ireland - Volume 1" (a jaunty little title, I know), and according to what I believe to be popular mythology, this beautiful tune / song goes back to Medieaeval times.
I have heard that the tune came to Ireland from the Middle East, via Europe, and also that it is a Gypsy melody. I can't say if either (or both) theories are correct though. I've always preferred to think that the tune wandered around continents in search of a home. Van Morrison's version is stunning.
Don't know if this counts as "information", but hope it's of interest.
Valerie