Hi Whiz
For holiday use the Revolut MasterCard and app is streets ahead of anything else. Couple of pointers:
1) You must be handy using apps on your phone. There is no web interface.
2) Apply for your card in plenty of time, and make sure it works in Irish ATMs/shops before you travel.
3) Use your current account debit card to load Euros, you get immediate value, bank transfers take a few days.
4) Use the app on your phone to convert Euros into US$ during Dublin/London business hours, in other words, when the foreign exchange markets are open. That way you get the Interbank rate. If you convert currencies when the markets are closed your rate isn't as good.
5) After your trip, convert unused US$ back into Euros, again, during working hours to get the best fx rate.
6) Have a backup credit/debit card and a small amount of US$ in cash, just in case.
7) Try to do your ATM withdrawals from major banks, like Bank of America, which doesn't apply a fee. Avoid ATMs in convenience stores.