In the north-eastern part of the lake, a small artificial island sits partly submerged, its stone foundations spreading out four metres below the waterline on both its northern and southern sides. This is a crannog, one of the lake's six known examples clustered in relatively close proximity, and one of the more distinctive forms of early Irish settlement. Crannogs were man-made or partly man-made island dwellings, typically constructed from timber, brushwood, peat, and stone, built out into lakes or wetlands to provide a degree of natural protection. They were in use in Ireland from the Bronze Age through to the early modern period.

The island itself measures roughly thirteen metres in diameter. At its centre, the ground rises gradually to a flat-topped mound standing about 1.2 metres high, a profile consistent with the accumulated remains of occupation material and deliberate construction. A low bank survives at the southern edge, a few metres long and a fraction of a metre high, and what may be a simple slipway, marked out by loose stones, is visible on the northern side. A possible causeway once extended eastward from the island, suggesting a deliberate connection to the lakeshore rather than purely water-based access. The interior is now densely overgrown with brambles, trees, and fallen timber, which both obscures and, in a practical sense, protects whatever structural or organic remains might survive beneath. The fact that this is just one of six crannogs in this part of the lake points to sustained, concentrated use of Lough Rea's waters across many centuries, though the precise dating and history of this particular island remain unexcavated.