This is a version of the very popular story of the beggarman who makes off with the daughter of the house and then turns out to be a lord or nobleman (as in The Whistling Gypsy).
The Clancy Brothers got this from their sort of second mother, Annie Roche. Annie had a store of old traditional songs which she never considered of interest to anyone until the ballad revival of the 1960s made her rack her brain for half-forgotten songs she remembered her mother singing.
