I'm going to assume you are using the term Celtic very loosely. There are some very nice books easily available from bookstores and libraries that contain lots of old traditional Christmas songs that aren't often heard by the general public. One of my favorites is The International Book of Christmas Carols by Walter Ehret and George K. Evans, published by The Stephen Greene Press. It contains more Christmas songs than you will ever be able to learn. Also look for The Christmas Revels Songbook compiled by Nancy and John Langstaff and published by David R. Godine. Then there's Three Log Night edited by Jane Peppler and published by Skylark Productions in Chapel Hill, NC. And, of course, The Oxford Book of Carols published by the Oxford University Press.

Here are some songs I've done over the years and enjoyed:
Blessed Be That Maid Marie
The Boar's Head Carol
The Holly and the Ivy
The King
The Salutation of the Angel
Chrisimis Day
Masters in This Hall
Babe of Bethlehem
Personent Hodie
Come Mad Boys, Be Glad Boys
Star of the East

John Peekstok