It is really an advent carol, but almost entirely unknown, I think. About its origins Mudcat.org says:
John Taylor, Postbridge aged 85, Tune not recorded in Killerton Ms but can be found in the Rough Ms. Baring-Gould gives a second version which he says is closer to the broadside ballad.
K3 p297
from Sabine Baring-Gould and the folk songs of South-West England
The gorgeous version I have is by Jennifer Cutting's Ocean Orchestra (
here you can hear small sample).
I found another artist who put the whole song online, giving it a different title.
( Here are the lyrics of the version I know )