"She has excellent children."
Translation:Tá páistí iontacha aici.
I am clearly struggling today. I can't understand why this doesn't use the copula. I just had the sentence 'is buachail x é,' (can't remember the 'x') and this seems like it should be the same construction. Is it because it is a possessive statement?
Yes, it's because it's possessive. Since Irish has no verb for 'have', it makes use of the phrasal verb bí ag, which doesn't require a copula. If you wanted to say "He is an excellent child", for example, you'd have to use the copula.
You could potentially say Tá clann iontach aici, with the singular form of the adjective, but it would suggest that her children are wonderful as a group, rather than as individuals.