Thanks to everyone for your help. I'm not sure that Sarah Makem's version is in the DT, but Roberto had posted it in this thread:
http://www.mudcat.org/Detail.CFM?messages__Message_ID=956755.

Let me clarify before we head off into the Barbara Allen Wilderness, that I'm not seeking the origins of the whole tradition or the song itself, but only precedents to Sarah Makem's version, Irish or otherwise.

The "BARBARA ALLEN 5" in the DT by Johnny Moynihan on the album "Selected Songs, Reels, and Jigs" by DeDanaan, is surely taken from Sarah Makem. I haven't heard this either so I don't know if the tune is the same.   T.O.M.