Solved!

I had the name wrong.
It is a villancico, and Loreena McKennit recorded it on her album "The Mask & The Mirror"
http://www.last.fm/music/Loreena+McKennitt/+videos/+1-s1AcESk7JBA

According to her liner notes she learned it from a CD herself.
Also a Portuguese friend recognized it as a Christmas song.
So I"m going with it.
Thanks, all!