The Rock of Cashel is a large ruined church on a small rocky outcrop in Cashel, Tipperary. While the site was originally a pagan site, with the arrival of Christianity, like many other things in Ireland, it was given a whitewash of the Church and was the seat of the Kings of Munster ever since St. Patrick converted the then king.
It has some the best examples of Celtic and early Medieval architecture in Europe and is often called "Ireland's Acropolis".
And yes that is my thumb as I was adjusting the camera....
Cahir castle, in the nearby town of Cahir, was built in 1142 by Conor O'Brien, Prince of Thomond. It ended up in the Butler family until the last ancestor died in 1961 when it was taken over by the government. It was up until then an actual residence. The Butler family actually lived there.
If it looks familiar, it was used extensively for the filming of the movie Excalibur.
The town of Cahir, like most areas with castles, sprung up around it.