Rock of Cashel
Located on a hill with a stunning view, the Rock of Cashel is a fortress with a large Romanesque church that dominates it.  Historically, the site dates to around 400, but the present ruins date from around the 12th century, when the church was built.  There is a rich history of clan and Church ownership here, and it finally fell to Cromwell's army in 1647.  It was briefly used as a Protestant Church in the 18th century, but fell into disuse after that.  It's such a beautiful site, that I recommend it be near the top of any list of places to visit in all of Ireland.