Will Apple Watch work in Ireland

Work in what way? If this is a GPS only Watch, it should work just fine, paired to your iPhone. If this is a GPS + Cellular, if it is not the European version, you will not be able to use it without your iPhone since the Watch cannot roam on cellular networks. Also, if it is not the European version, a European carrier cannot activate the cellular on it. So, depending on exactly what you are asking about it working, the answer could be yes or no.

So long as you have your paired iPhone with you, there should not be any issues.

You're welcome.