What remains of the cashel, a type of ringfort built from stone rather than earth and timber, amounts to a spread of collapsed drystone rubble tracing an arc from the northern side around through the east and down to the southwest. The structure is roughly twenty and a half metres in diameter, and where the original wall has not survived, later field walls have effectively taken over the job of enclosure, following the monument's circuit from the southwest around through the west and back to the north. Two of those later walls also cut across the monument itself at the northwest and southeast, complicating the picture somewhat.

Cashels of this kind are characteristic of the early medieval period in Ireland, roughly the fifth to the twelfth centuries, and would originally have served as enclosed farmsteads, the stone wall providing both a boundary and a degree of protection for a household and its livestock. The fact that this one has been absorbed into the local field system is not unusual. Across much of the west of Ireland, ancient enclosures were quietly pressed into agricultural use over the centuries, their walls robbed for new boundaries or simply left to collapse as the land around them was reorganised. At Keernaun, the two functions, ancient enclosure and working field boundary, have become so intertwined that the monument is partly defined by walls that have nothing to do with its original construction.