Just for the record: No, it's not part of England ;-)
The Northern Ireland (its capital Belfast) is part of the United Kingdom of Great Britain & Northern Ireland, alongside with England, Scotland, and Wales.
Dublin is a capital of the Republic of Ireland, which is on the south, and is a separate country (TLD .ie :) which uses euro as the currency, contrary to the UK's pound.
I am being a bit pedantic with since the whole UK/Ireland story is still a pretty fresh wound and making such a mistake might get one some negative brownie points in the conversation :-)
It's a bit like making a joke about Israel/Palestine.
In Northern Ireland at least, you had state-sponsored terrorism by the police, IRA blowing up civilians, thousands killed - the very worst sectarianism imaginable. Some of this continued into the 90s so yes, the wounds are still fresh.
That doesn't even delve into the long, brutal history between Ireland and England spanning centuries, a relationship that's only healing today.
But good catch... I meant countries where English is not already the native language.