You must have a valid passport to enter Ireland. U.S. citizens can enter visa-free for tourism or business stays of up to 90 days.
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Passing Border Control means entering Ireland, and you are not allowed to enter Ireland without a visa unless you are visa-exempt. Anyone who is subject to Irish visas and has to pass through Border Control during their transit needs to have an Irish entry visa (Ireland short stay “C” visa).
Please note that Ireland is not party to the Schengen Agreement, and a Schengen visa or residence permit does not entitle you to travel to Ireland without a visa.