The island itself may have been artificially enlarged to support the fortress, a detail that shifts the scale of ambition behind this remote stronghold from the merely impressive to the quietly extraordinary. The eastern side of the wall has been largely levelled, probably where the original entrance once stood, and the interior offers little in the way of obvious features beyond a small beehive-shaped hut against the north-east wall and two rectangular depressions sunk into the ground.

The structure is thought by researchers Naessens and O'Conor to be a possible pre-Norman fortification, dating to the eleventh or twelfth century, though it may have begun life as a cashel, a type of dry-stone ringfort, before being reinforced with lime mortar at a later stage. The first written record appears in the Irish annals under 1195, when Cathal MacDermot O'Conor used the island as a base while plundering Connacht, committing further raids across the surrounding territory from this lake-bound refuge. The fortress reappears in the record nearly four centuries later, in 1586, when followers of the Burke family were driven here under pressure from Richard Bingham, the Elizabethan governor of Connacht whose campaigns across the west were notably brutal. It has been suggested that the upper portions of the enclosing wall were added or modified around this time; the masonry style and wall thickness visibly differ between the lower and upper sections, pointing to at least two distinct phases of construction across several hundred years of use.