The James Stephens translation is little different from the other versions quoted. Does anyone have access to Irish words which might clarify what's really going on in the story?

It seems very likely that "earl" is a mondegreen, but that would kybosh the droit de seigneur explanation. (And did that practice ever exist in Ireland anyway?) The Sean O'Boyle explanation, that drink has rendered the husband impotent so that the wife prefers a younger man, is somewhat more plausible. Asking whether she is the daughter does indicate that the husband is a lot older, and there are other songs about cuckolds including "never wed an old man".

I suppose the air of mystery helps to explain the attraction of the song.

There's an inconsistency anyway between the husband apparently being perfectly willing to "lend" the wife but then reacting so badly when she comes back.

I suppose we have to presume that it's his ghost telling the tale.