The "Holly bears a berry" stuff is the "Sans Day Carol", from Cornwall. It's in the Oxford Book of Carols (the old one, not the recent one, though it may be there, too). "Sans Day" is a dialectal form meaning "St. They", referring to a semilegendary saint popular in Britanny and Cornwall. The text was originally in English but has been translated into Cornish (and, I believe, an original Cornish verse appended). The William Auld/Margaret Hill Esperanto version is on my website.

The holly-and-ivy imagery does I think reflect prechristian traditions, though none of the carols we now have containing it goes back very far.

Liland