I think they were custom (they matched very well), but I haven't been able to confirm who the designer was.
Here's a photo of their act 1 dresses though from a photo shoot at Johnstown Castle (probably the morning of show #2).
I borrowed the photo from the Celtic Woman Brasil fan page on Facebook.
Page:
https://www.facebook.com/Celtic-Woman-Brasil-156278887741179Photo:
https://www.facebook.com/156278887741179/photos/pb.156278887741179.-2207520000.1538708492./1885097821525935/?type=3&theater