If reserving from the states (on a U.S. site) as pay now, and have the choice, pay in dollars IF your credit card has a foreign transaction fee. If it doesn’t have a foreign transaction fee, it shouldn’t matter. If the reservation is reserve now, pay at the hotel, select/pay in the local currency; in your case, Euros. If you elect to have a hotel bill (or any bill or restaurant check) charged in dollars while in Ireland, you’ll get a worse exchange rate. In Stratford upon Avon last May, a restaurant charged me in dollars without asking, and the meal cost about $2 more than it would have had the waitress charged the meal in pounds because of the exchange rate that was used. While not much, if added up over the course of a trip, it could be quite a lot. That waitress got zero tip since we generally leave a couple pounds.