Either from England or Scotland; other source extremely unlikely. Check out
Wikipedia entry (though you should ignore the suggestion there that the
ballad might have originated in Ireland, as none of these international
ballads apparently did so.) Pepys's diary refers to it in the 1600's as
Scottish, but many of the variants give the locus as Reading or London. As
for the 'mutation,' put it down to the vagaries of the oral tradition. There
are a lot of versions of Barbara/Barbro/Barb'ry Allen/Ellen to be found in
Ireland, and tracing the origins of one of them simply isn't possible.
What version is it that you have that begins "Christmas comes but once a
year"? I have eight versions of this song in my library, but can't find the
one with that beginning. Here's what I do have:
"It being about the Christmas time/when green leaves they were were falling"
"It was early, early in the summer time / when the flowers were freshly
springing" (Elizabeth Cronin)
"It was in the merry merry month of May / as the rose-buds they were
swelling" (Patsy Flynn, second verse)
"Michaelmas Day being in the year / when the green leaves they were falling"
"Springtime comes but once a year / when all things are in blossom" (Thomas
"One day, one day of a fine summer's day / it was all (?) green grass was
growing" (Andy Cash)
"Christmas came and went one year / The green leaves they had fallen" (Joe
Heaney, second verse)
"It was early early in the spring / when flowers they were blooming"
Hope this is some help to you.
"Ever notice that 'what the hell' is always the right decision?"
-- Marilyn Monroe