Euros

As always, if your bank does not charge you a Foreign Transaction Fee (FTF) and/or an "Out of Network" fee, then using an ATM is almost always the cheapest. You get about a 1% bump over the exchange rate you see on the internet as the "Interbank" rate. You may also pay a use fee at the ATM, a few euro.

Getting euros here is at best about a 5% premium, typically 7%. Exchanging physical cash is about the worst way to do it, probably in the range of 7-10%, possibly more.

Edit: Should add, you will use mostly a credit card though, not much need for lots of cash.

Just use credit cards.

remember if you are visiting Northern Ireland as well as Ireland you will need UK pounds
Northern Ireland is part of the United Kingdom and uses Pounds not euros