The most popular day trip from Dublin for American tourists is the Cliffs of Moher and Galway Tour. It’s the most referenced tour in TripAdvisor ratings and reviews.
It’s popular with Americans because it delivers a memorable « Ireland » experience in one day: huge Atlantic cliffs, green countryside, tiny villages, Irish history, and Galway, a city renowned for its culture and arts.
It’s a long tour, taking between 12 and 13 hours but it does cover a wide variety of experiences and is highly rated. Wild Rover Tours is the top-rated day tour operator to Cliffs of Moher and Galway.