The first I heard of this song was from Dominic Behan who said that he had learned it from the actor Peter O'Toole, whose favourite song it was. Dominic recorded it in the early sixties. His words differed slightly from those sung by the Clancy's whose source is surrounded by the usual creative mist. I have never since heard any version which could not be traced to either of those, nor have I ever - and I have looked at most of the manuscript, ballad sheet, popular song book or book collections of Irish songs ever made - seen it in any prior form.

It is, as you say, a mystery. I'll give it some more attention.