The island itself, measuring roughly 60 metres east to west and 50 metres north to south, sits between 250 and 450 metres from the surrounding shorelines. Every stone used in its construction had to be ferried across from the mainland, with a submerged slipway still visible at the southeast corner connecting to another slipway on the shore some 280 metres away. The southeast quadrant of the island may even be an artificial construction, expanding what nature provided.

The Rock’s recorded history begins in 1159 when Diarmuid Ua Maelruanaidh died there, though its most dramatic early moment came in 1184 or 1187 when lightning struck the island, causing a catastrophic fire that killed Duvesa, wife of Conor Mac Dermot, along with over a hundred others who burned or drowned. Throughout the Middle Ages, possession of the Rock was fiercely contested; it was besieged by the Justiciar Maurice FitzGerald in 1235, only to be levelled by Conor Mac Dermot upon recapturing it, burned by Cathal O’Conor in 1321, and fought over by different branches of the Mac Dermots themselves in 1401, 1478, and 1488. The castle’s finest hour came at Christmas 1540, when Rory Mac Dermot and his wife Sadhbh Burke McWilliam hosted lavish entertainments here. The tower house that forms the core of the present structure may be the ‘great regal house’ begun by Brian Mac Dermot in 1578.

Recent archaeological excavations have revealed the site’s complex evolution, finding evidence of an early cashel wall beneath the current enclosure, sealed by a layer of burning that likely corresponds to the fire of 1184. Above this, medieval buildings with drystone walls and clay floors were constructed. The oval enclosure wall, though heavily rebuilt in the 19th century when the King family transformed the ruins into a fishing lodge possibly designed by John Nash, incorporates earlier masonry. The tower house at its core, with walls two metres thick, contains original features including a garderobe chute and a blocked window with an external chamfer. After surrendering to Cromwellian forces in 1652, the Rock passed to the King family, who consolidated and extended the buildings in the early 1800s, adding the crenellations that crown the walls today. Now a National Monument in state guardianship, Mac Dermot’s Castle remains one of Ireland’s most evocative island fortresses.