I've tried this a few times and it's always the same. If you check flights on United from Dublin to Newark the price is about €65 higher if you have an Irish address rather than an American address. Same flights, same dates, same everything, but for some reason the price varies wildly depending on whether your credit card is Irish or American.
I tried going out on Feb 18 and returning on March 9. With "Ireland" selected as my address the price of the flights was €482. If I change the country in the top right the price is $566, which is €415. I cannot understand the difference. All I can say is if you have an American credit card, use it if you're buying a ticket to Newark.
Oddly, I don't see any appreciable difference in price if you put it another destination other than Newark. I have no idea what's going on here.