Can some give me a breakdown of who’s singing in which songs? It’s kind of tough for me with Sibeal
Sure! I’m rather unsure on some of them, though, so maybe this isn’t the best help.

It’s especially hard for me to tell if Tara/Ciara is on there as the Brian Byrne arrangements tend to blend her in too much with the other background instruments. Muirgen and Caitríona have somewhat similar voices as well, so it’s difficult to tell between them.
God Rest Ye Merry Gentlemen: group
Nollaig na mBan: group featuring Sibéal
Have Yourself A Merry Little Christmas: group
River: Mairéad solo (I believe it’s just her, but Tara/Ciara might be on there as well)
Codail A Linbh: group featuring Sibéal
Silent Night: instrumental (I’m afraid I don’t know if it’s Tara, Ciara, or both)
Wexford Carol: Muirgen singing with Tara/Ciara on fiddle
Little Drummer Boy: group
Don Oíche Úd i mBeithil: group featuring Sibéal
Silver Bells:

It’s either a group, just the vocalists, or Caitríona and Mairéad together. Caitríona and Muirgen have very similar ranges so it’s hard to tell for me. It’s also odd that Caitríona didn’t get a solo.
In The Bleak Midwinter/Goin’ Home: group (I think?)
The Bells Of Dublin/Christmas Eve/Navan In The Snow: instrumental featuring Tara Howley, either Tara, Ciara, or both.
Auld Lang Syne: group
(sorry if this was all wrong!)