I checked every resource I could think of, with no luck. I thought of the York Christmas Play or the Shepherds' Play, but that didn't pan out - couldn't find all the plays, so that's a vague possibility.
-All mention of the song points to none other than Jean Ritchie. It's a haunting tune - I really like it.
So, either it was written by Jean Ritchie, or it's "traditional" (I understand that means Sandy Paton wrote it).

-Joe Offer-