This rectangular four-storey tower, measuring roughly 12.3 metres long and 10.6 metres wide, dates back to at least 1574, when records show it belonged to one Moyler McShean. Today, thick ivy cloaks much of the structure, and centuries of stone robbing have left it in poor condition, with a significant breach visible in the east-northeast wall.

Despite its deteriorated state, the castle still reveals fascinating defensive and domestic features. The original entrance on the north-northwest wall once led visitors through a small lobby overlooked by a murder hole; a grim reminder of the tower’s defensive purpose. From here, a short passage connects to spiral stairs that would have provided access to the upper floors. The first floor retains evidence of daily life, including a garderobe built into the wall and a fireplace whose flue can be traced up through a window embrasure on the second floor. Stone vaulting separates the ground and first floors, whilst the second floor contains intramural chambers within the north-northwest and south-southeast walls.

The castle’s defensive architecture extends to its roofline, where projecting corbels above the doorway mark where machicolations once allowed defenders to drop objects on attackers below. Various window types survive, from defensive slits to flat-headed rectangular openings. The immediate landscape suggests a larger medieval complex: approximately 40 metres southeast lies a holy well, whilst just 30 metres east, earthworks indicate what may have been a moated site, hinting at Cloonboo’s former importance in this corner of north Galway.