"would it solve your horticultural problem"
Not really a problem Anne - I enjoy the song wherever and whenever it took place
Just commenting on a n aspect of the text which has often occurred to me in response to the suggestion that Christmas was a corruption, implying that one was correct - in this case, neither makes sense, and we don't know which was original.
What interested me was that I had't come across the Christmas reference in Irish versions other than those from the Gealachts and I wondered if there was any significance.
Quite often, reference to times of day or year in ballads and songs carry with it folklorists, social, or local information, or sometimes they are scene setters to create a backdrop for the singer - I believe they are seldom merely rhyming conveniences.   
We got this from Sam Larner's nephew, who sang us, 'Just as the Tide Was Flowing' and explained what that time of day meant to a fisherman
Jim Carroll