Nollaig na mBan (feat. Sibeal) is released.
It shows up on my YouTube and Spotify for example.
It's beautiful!!!
Thank you so much for letting me know! I’m listening to it right now.  

 It seems as though they’ve given Caitríona the first part, but it might be Sibéal (I haven’t heard much of either to be able to tell 😬). Either way, the opening lassie sounds absolutely majestic! Muirgen and Mairéad are incredible as well  

 I think this will end up being something I’ll listen to all year round  

.
I am surprised the song isn’t in Irish. The title comes from one of the traditional twelve days of Christmas, and apparently is the title of another song by someone named Jennifer Licko, and although I haven’t listened to that yet, given that it took me three pages on Google to find it, I’m going to assume this one is a CW original.
ADDENDUM (because I was too lazy to edit the first paragraph  

): I do believe after listening to some Sibéal (her rendition of The Wind That Shakes The Barley, which I would recommend! It’s quite captivating!) that it was indeed Caitríona in the beginning. Her voice is utterly enchanting and fits perfectly with CW. 
Also, I’m glad to finally have an answer to what song they were singing in the promo video that went “winter’s daughter with her tender song”, because that was driving me mad trying to figure out what it was! 😭