The berth, built to receive boats at the north-western entrance of the structure, was left stranded when the lake level was deliberately lowered in 1965, and it now opens onto dry land. That detail alone gives the place an odd, slightly suspended quality, as though the water simply forgot to return.

The enclosure occupies the south-western end of Illanee, also known as Garrison Island, roughly 250 metres from the western shore of the lake. It is built in the manner of a cashel, a term for a stone-walled enclosure of early medieval Irish origin, though here the stones were bonded with mortar rather than stacked dry. The internal dimensions are considerable, around 26 metres north to south and 24 metres east to west, and within the walls a possible two-roomed structure sits in the north-eastern quadrant, with a small D-shaped hut pressed against its south-eastern gable. Much of the mortar has eroded over the centuries, and sections of the enclosing wall have collapsed as a result. Researchers Naessens and O'Conor, writing in 2012, identified the monument as a possible pre-Norman fortification, placing its origins somewhere in the eleventh or twelfth century, a period when Irish lords built island strongholds as much for control of waterways as for defence.

Accessing the island requires crossing the lake, and the now-landlocked boat berth serves as a reminder that the whole arrangement, island, enclosure, mooring, was once designed around movement by water. The collapsed walls are substantial enough to read clearly in the landscape, and the internal structures, though ruined, retain enough form to suggest how the space was once divided and used.