It measures roughly forty metres long, twenty-five wide, and rises only a metre and a half above the surrounding ground, which means it would be easy to walk past without registering it as anything other than a slight rise in a soggy field. In fact, it is a crannog, one of the distinctively Irish and Scottish form of artificial or semi-artificial islands, typically built on lake shallows or boggy ground during the early medieval period and used as defended homesteads or places of refuge. This one is poorly preserved, its original form blurred by time and the slow encroachment of the marsh.

Crannogs were constructed by piling up layers of timber, brushwood, peat, stone, and earth, often anchored with wooden stakes and sometimes ringed with a timber palisade. They were occupied from the Bronze Age through to as late as the seventeenth century in some parts of Ireland, though the early medieval period, roughly the fifth to twelfth centuries, saw their most widespread use. The setting at Cartronroe is characteristic: open water to the north, running water to the south, and saturated ground making any approach on foot difficult and slow. That natural defensibility was the point. The earth and stone that remain suggest the artificial platform has largely slumped and spread over the centuries, leaving the pear-shaped outline as the main evidence of what was once a deliberate and inhabited construction.