Annraoi and Mick, I decided to do a search of the internet using lúibín instead of luibin, and Dogpile returned many, many sites, including the lyrics to another song in which the word lúibín is found. It does not fit the tune of the one I am looking for, but I thought you may find it interesting.
Úna bhán - Fair Una - (Blond Agnes)

Still looking for Lúibín O Luth.