Roughly twenty metres east to west and eighteen and a half metres north to south, Cartron Island is built, at least in part, by human hands. Its edges are defined by a stone platform, and the interior rises gently to a flat mound, suggesting deliberate construction or modification rather than purely natural formation.

What survives on the island points to a settlement of some kind, though its age is not precisely recorded. The interior holds the remains of two stone huts, and there are traces of drystone revetment along the northern edge, where courses of unmortared stone were laid to retain or reinforce the mound. The whole thing is now heavily overgrown. Islands of this type in the west of Ireland often belong to a tradition of small lakeshore or lake-island occupation that stretches back across many centuries, sometimes associated with crannogs, which are artificially constructed or artificially enlarged islands used as defended dwelling places, and sometimes with later monastic or pastoral use. Cartron Island does not fit neatly into any single category; it is simply there, compact and quietly anomalous, a stone platform carrying the outlines of two small rooms in the centre of a Connemara lake.