Nos Galen was published in "British Harmony," 1781, John Parry, as one of 42 tunes with Welsh titles. Some claim great antiquity for the tune, but no supporting evidence. It could have other names (See Skipper Jack).
http:// web.ukonline.co.uk/mustrad/articles/kid_txt2.htm
Also in Welsh, the Night of the Winter Calends (Halloween) is called Nos Galen gaeof.
Without seeing Parry's book, I don't know if the tune is the same as "Deck the Halls." No lyrics are given.