Buy Euros

In Ireland, as in every European country, by far the least costly way to obtain local currency is from ATM's, using your own ATM/debit card from home.

ATM, always.

I buy Euro from my local credit union. They charge the daily rate, plus .50 to mail the letter to get them. Takes about 5 or 6 days to get them back. That way, when I land, I have Euro for cab, food, and first few days. When I run low, I then go to the ATM and get more. DO NOT FORGET to tell your bank that you are traveling and also increase your daily limit for withdrawals. Also, you cannot get money from saving, etc. in you bank account. You can only get cash from the checking account the ATM card is tied to. Lastly, you pin must be 4 digits only. If you go to Doolin, there is NO ATM. Irish ATM do not charge a fee, most US banks charge fees, fees and fees.

ATMs are definitely the way to go. That being said, we did just purchase some Euros through our bank as I am one of those travelers that wants some local currency in my pocket when I arrive. The bank "daily rate" is likely higher than the actual exchange rate. To give an idea, when we bought ours the actual rate was 1.32, but the bank's daily rate was 1.41. Yes, I know we paid more, but for convenience and peace of mind it was worth it. We have enough for day 1 and will use ATMs the rest of the time. Enjoy your trip to Ireland! We are going in April and can't wait!! :)

I agree with buying some before you go.

There are ATMs in every commercial airport in Ireland. That is the cheapest way to get euros. I can't see any reason you would need to buy any significant amount of euros before then. I agree it feels nice to land with a little bit of local currency but it's not necessary. If having a few euros is important to you, you can change $50 or $100 at the airport. That will be the worst possible exchange rate but for the amount of money involved it's irrelevant. If you do that, ask for as many euro coins and 5 euro bills as you can get. Initially, having smaller denominations will come in handy. Once you start spending money, you'll quickly be overloaded with coins and you'll start working at paying for things with coins. Change and smaller bills are usually not as easy to come by when you first land since ATMs generally spit out 20 euro bills and larger. Change is especially useful for tolls and parking. You can't get them from an ATM or US bank. If you know you'll need change for a toll immediately after leaving the airport, get change from a coffee shop in or around the airport. If necessary, buy a small snack. Otherwise, don't bother. Once you go anywhere in the euro zone, keep a few euros worth of coins and a few small bills for your next trip.

Definitely ATMs.
And don't buy any before you go. Unless you're completely tweaking about landing in a country without the currency (there are ATMs and banks in the airport, remember), don't do it. You'll get robbed on the exchange and commissions.

Yes ATMs. Now that said, you bank will have a say about extra charges and/or exchange rate markups. For example, Bank of America will charge you $5 USD plus what ever the ATM operator charges. However, BofA waives the $5 fee if preferred banks are used. Some non-bank ATMs may have very high usage fees.

To answer your question 'What is least costly way to get Euros in Ireland?" An ATM, since you're in Ireland there is no way you can get them at a bank in the U.S.

if you buy them here in the US get them soon. other tourist may also be planning spring trips to europe. some banks need up to 15 days to get you a supply. if you buy over 1000 euro the exchange price may go down.