I think what is occuring is Eiméar somehow has a similar tone like Caitríona when they are together and what makes Caitríona drowned out as well is if she is doing the lowest areas at times also, I cannot tell but it's hard to even tell if Muirgen is swapping, if ever Muirgen swaps anything she's so noticeable.
I know blending is the gift, but maybe raise Caitríona voice so she soaks in like Eabha because she has that unique irish flair.
On God Rest Ye Merry Gentleman you could ignore and listen as if Lisa Lambe was slowly on it
What's crazy to me now is when Eiméar does the high soprano parts sounds very Mairead Carlinesque

I am still getting to know Eiméar and there were comments saying they miss when the ladies sang each individual parts of the songs.