Jim -- I'm fairly certain that the Breton tune to which you refer was, in fact, the tune attached to "The Twa Corbies".
I heard Ray and Archie sing this song often in the early/mid-60s and (as I remember it) the introduction moved from night visiting into bundling customs, with no real mention of source. It was certainly a very popular song at the time, probably because it had a sweet tune which was easily harmonised (for those who wanted to) and could be accompanied readily with a simple guitar ripple.