Locals call it Rocky Island, but it is formally recorded as a crannog, one of those artificial or semi-artificial island dwellings built in Irish and Scottish lakes from the Bronze Age through to the early modern period. What makes this particular example quietly peculiar is the water around it. Crannogs were typically constructed in the shallower margins of lakes, where building on timber piles or piled stone was manageable. Here, the water reaches depths of up to 4.5 metres, which is unusually deep for this kind of structure and suggests a deliberate, effortful choice of location.

The island itself measures roughly 18 metres north to south and 15 metres east to west, rising about 5 metres above the lake bed. It was constructed on a natural rocky knoll, with the builders piling medium to small-sized stones into a cairn that has a notably level upper surface and an even, symmetrical profile before dropping steeply into the water, most sharply on its southern side. Around the base of the structure on the lake-bed, vertical roundwood posts have been recorded, the kind of timber framing commonly associated with crannog construction, used to consolidate and retain the mass of the cairn. O'Sullivan, writing in 2004, noted that the islet would actually have sat below the waterline before drainage works lowered the level of the lake, meaning that what is visible today is the result of nineteenth or twentieth-century land drainage rather than the original builders' intentions. At the northern end of the islet, a modern duck-hide or low stone kerb wall has been added, a prosaic footnote to a structure with considerably older origins.