I was going to suggest "The old year now away has fled" (I prefer "is fled") but GUEST beat me to it. However, she didn't mention the tune, which is Greensleeves, one of the finest older English melodies. More verses. As for Nos Galan words, I rather like the "All ye mountains praise the Lord" version in the old Oxford Book of Carols, not sure if it's got a Cymraeg substrate.

Haruo