Cool name choices!
Carolina and Ireland strike me as being opposites -- one is girlish, the other not-so-girlish. So I think what I would do is use the middle names to balance out the first names a bit, so both girls have a girlish name and a not-so-girlish name. Just to be fair to both.
For Carolina, I think Presley, Rowan, Ascher and Lyndan all work. (I do love the name Isabel, but I wouldn't use it in this case.)
For Ireland, I like Giselle best of the options you listed. And I think I'd look for other classically feminine names as well -- maybe Camille, Fiona or Claire.
Personally, I'd stay away from noun-names like Snow and Gray. To me, "Ireland Snow" sounds like something in a weather report. "Ireland Gray" sounds like a paint color. I wouldn't want other kids to pick up on this and start teasing her.