"Mondegreens"
I don't think it was a Mondergeen.
"Do you think that "Christmas" is perhaps derived from "Martinmas"?"
Seems a big poetical jump, but who knows.
Martinmas is certainly an important period in the Catholic calendar, and also folkloristicly here, so there's no reasons for it being rejected or mistaken because it was unfamiliar - one of the old customs in the West was to scatter the blood of a newly-killed animal around the four corners of the house on 10th November.
Irish Travellers, who played an essential part in the transmission of ballads, would certainly not mistake it, as "Blessed St Martin" is their patron saint (when we were recording them, a couple of them referred to him jokingly as "Cassius Clay" because he was black and his popular image shows him posing like a boxer, with his fists together in front of him)
Thanks for that Jim (Brown) - pretty convinced that the version was circulated in the area by Vail Ó Flatharta - an extremely impressive singer.
Barbara Allen is the only song in English on his album. 'Bláth na nAirní
Jim Carroll