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.