It is not a natural island. Someone built it, piling stone around an existing rock outcrop to create an artificial platform measuring around 28 metres north to south and 22.5 metres east to west. Heavily overgrown now, it reads at first glance as nothing more than a scrubby islet, the kind that dots western Irish lakes in considerable number. Look more carefully and the geometry of the thing gives it away.

This is a cairn, and its closest parallel sits just to the north: a crannóg, the term for an artificial island settlement, typically built from timber, brush, and stone, and used throughout prehistoric and early medieval Ireland as a defensible dwelling place on water. Otter Island appears to be a related structure, formed around a natural rock base and reinforced with stone rather than the more typical timber-and-peat construction. A short breakwater, around four metres long, extends to the southwest, suggesting the builders were managing water approach carefully. Beneath the surface on the southern side, timber posts are still visible, hinting at earlier wooden elements or structural supports now submerged. What purpose the cairn served, whether as a habitation platform, a storage site, or something associated with the adjacent crannóg, is not recorded.