Clonmacnoise was founded by St. Ciaran in the middle 6th century and monastic settlement. There is a cathedral, round tower and several churches (including 1 little temple where Ciaran is buried). Pope John Paul II once here and there is a building commemorating that event.
This site is most accessible by car and is a 2-hour drive directly west from Dublin and located on the River Shannon. A good map is a necessity so you don’t get lost on unnamed country roads.
First walk through the museum in the Visitor’s Center to see the original high crosses (the ones in the settlement are copies) plus some stone carvings which appear to have been tombstones. There is also an audio-video presentation about the settlement which was quite informative.