We didn’t get inside Grennan Castle, but did catch a glimpse on one of our castle-hutting-near-Killkenny days. This castle was constructed in the year 1210.
“Grennan Castle measures 20 metres x 13 metres and stands 20 metres tall, the walls at their base are 2.5 metres thick. The ground floor contains three barrel vaulted chambers with an entrance to the second floor on the south side by a staircase built hidden into the thickness of the wall. At the top of the staircase is just a gaping opening down to the floor below. This opening was originally covered by a trapdoor, hence with the trapdoor raised the castle was almost impenetrable.”
Abandoned Ireland also notes that “In 1650 Cromwell’s troops called to the castle. The siege lasted only two days, the garrison marched out leaving all their weapons behind them and promised to never again oppose English rule.” It wraps up by stating, “Grennan Castle survived in good repair until the early 1800s.”