Originally Posted by Anthemyst
If you both enjoy being active, biking up to the tops of the cliffs of the Aran Islands is fantastic.
I can't remember the castles but there were several good ones along the east coast. I second the Aran Islands and cliffs of Moher. Other memories that stuck with me were the Giants Causway (a bunch of hexagonal pillars going out into the ocean, one of the cooler natural formations i've ever seen) and Saint Oliver's Plunkit's head that can be found in a church in Drogheda (heard the story growing up but assumed it was fake).