Visit a local bank (Bank of Ireland, AIB, Ulster Bank) or a reputable bureau (Moneycorp, ICE Currency Exchange). Bring your US dollars and a valid passport. Rates and fees are displayed at the counter.
Many Dublin ATMs accept US debit/credit cards for euro withdrawals. Watch for foreign transaction fees and exchange-rate markups from your card issuer.
Services like Wise (formerly TransferWise), CurrencyFair, or OFX allow remote conversion, often with competitive rates and low fees. Research thoroughly before using.