[nebulous1]

> The US says if X is the USA then you are NOT a citizen of the US

That is not correct. In the majority of cases the child will be a US citizen at birth, and the parents do not have to bring the child to the US.

> Once you're 18, you are just another foreigner if you didn't already start the process.

Also incorrect. Even if no application is made until the child becomes an adult, application can still be made for derivative US citizenship.

> if Y is Ireland you probably aren't

Again incorrect, assuming by "settled in Y" you mean gained permanent residency.

[tialaramex]

I evidently had this completely wrong, I can't now correct it, so I've upvoted those pointing out that it's wrong.