Exchanging dollars for Euros

Consensus of opinion here is to buy a couple hundred dollars in euros before you go, then hit the ATM when you arrive. You will probably find that the fees you pay to order them from a bank will be as much or more than just exchanging them when you get to O'Hare. The exchange rate is best when using local ATMs to withdraw cash, rather than buying them here.

For yrs I have just gotten my euros at airport atm's or airport banks in Europe. Never had a prob in any of europes airports. But if you are the super cautious type like I used to be then of course order form your bank in advance or use the international bank at your major international airport here in USA.But be aware airport banks here are not usually 24 hours so ck ahead.
Yes there are fees and a lower xchg rate but that is the price of xchg in USA.

For yrs I have just gotten my euros at airport atm's or airport banks in Europe. Never had a prob in any of europes airports. But if you are the super cautious type like I used to be then of course order form your bank in advance or use the international bank at your major international airport here in USA.But be aware airport banks here are not usually 24 hours so ck ahead.
Yes there are fees and a lower xchg rate but that is the price of xchg in USA.

Where in Ireland are you going to? In Northern Ireland, I believe the ATMs will give you Pounds Sterling. In the Republic of Ireland, the ATMs will give you Euros.

I never buy in advance, rates are horrible
I always use ATM when I arrive, rates are better

Buying Euros in the U.S. is kind of like buying groceries at the 7-11. You will pay more for the convenience and you probably don't want to do your weekly shopping there. I usually arrive in Europe with between $100 and $200 worth of local currency. I do this purely for my convenience. I like to have enough cash to cover the trip from the airport to my hotel and a meal without having to worry about finding an ATM at the airport. If I see an ATM as I'm exiting the airport, I get more cash. If not, I just wait until I am in the city. Plenty of people arrive with no local currency and manage fine; they use the ATM on arrival at the airport. I've only had one occasion where an ATM did not work for me on arrival (it was in Verona, Italy). Because I had some Euros on me, it was no big deal. That is part of the reason that I usually travel with some local currency.

For many 1st time travelers, having some Euros in advance is a comfort level thing. Some, as we did on our 1st visit, had to have cash right away. Ours was for cooking lessons. We couldn't be sure that the ATMs would be flawless and give us all of the cash we needed. In fact, it did but nobody on earth could guarantee us that they would. Our comfort level was just fine having Euros in our pocket as we got off of the plane. We now have a small supply of Euros, Swiss Francs, Pounds Sterling, Canadian Dollars, Mexian Pesos, Costa Rican Colones and Chinese Yuan for our future visits. Great stuff.

The others have covered this well, but a couple other tidbits for you to know: To buy euros in the US costs anywhere between 5-10% above the actual exchange rate that day. You'll pay that either in fees or a poor exchange rate or both. To use an ATM to withdraw euros, you'll pay 1-3% above the exchange rate plus whatever fee your bank charges for out-of-network. Europeans banks do not charge any fees. Check with your bank for their terms. Some people go so far as to set up new checking accounts that have no fees, but you can also often have your daily withdrawl limit raised to reduce the fees you pay.