I would suggest looking into some of works of Dennis Doyle. You can get his "Celtic Songbook" in a few days and though its not a wedding songbook it contains many appropriate tunes such as St. Patrick Breastplate (Lorica). You may want the CDs "Comtemplative Celt" and the "Abbess" to accompany the book.

Check out:

http://english.glendale.cc.ca.us/irish.med.html