I don't disagree with you Steve, regarding the potential mechanisms of confusion.

However it does seem to have happened in at least 2 different forms - one with just the "penances" being attached to what is to happen to the protagonist in a song that is otherwise clearly Child 20, as noted by Child and expanded upon by Mary Diane McCabe, who pointed out over 30 such examples from Scotland, Ireland (? - via expatriates) and Canada; the other being a mixture of the refrain of Child 20 along with (some of) the text of Child 21, as per the (presently unique) Thomas Moran version, a different kettle of fish it seems to me...

A separate question is from where the Irish "song" - I think we can call it now that as opposed to just a "version" of Child 21 - known as the Well Below The Valley derives. To my eyes it seems to have more in common with the continental variants or precursors of the "Magdalen Ballad" than with any Scottish versions that have come down to us, or the Percy Text which was the main source for Child - Tom Munnelly states (whether or not one can take this at face value) that John Reilly was aware that the song was a Magdalen legend, and in another version the stranger is revealed to be Christ as mentioned above.

Just some rumination on my part, but interesting that John Reilly's documented performances turn out not to be the outliers originally thought!

- Tony