Edward Bunting published a tune called Sweet Portaferry, and it is also given in Cathal O'Boyle, "Songs of County Down" Skerries, Co. Dublin: Gilbert Dalton, 1979
I haven't played the tune yet to see what it's like; I can't read music without an instrument other than voice. Jane Cassidy credits Gilbert Dalton (publishers)on her album so this book seems to be her source for the lyrics.

O'Boyle's notes: "In 1840 Edward Bunting published collection of Irish Music in which a tune called Sweet Portaferry appeared. I spent a day looking through the Bunting Papers in Queen's University Library, but though I found many Gaellic songs from Connacht, written down by Patrick Lynch of Loughin-island, I could find no record of Sweet Portaferry.
"I was beginning to tell this sad story to Thomas Pollard when he interrupted me, saying: 'Oh! I wrote down a song by that name from an old aunt of mine. Would you like to see it?' I don't think I ever finished telling my story, but I here present his words and Bunting's tune, and I think you'll agree they are a good match."

[tune to follow, I hope]