Roughly half of the enclosure extends directly into the water, so that where most ringforts rely on a fosse, the circular ditch that typically rings the outside of the earthen bank, the lake itself does the work on the western side. There is no ditch where the water begins; the bank simply drops into it. This kind of partial submersion gives the site a quality quite different from the typical rath set neatly in dry pasture, and it raises obvious questions about how the shoreline has shifted, or the water level risen, in the centuries since the fort was in use.

A ringfort, or rath, is the commonest type of early medieval settlement monument in Ireland, usually consisting of a raised circular area enclosed by one or more earthen banks with an external ditch. At Corwillick, the enclosure measures about 25 metres in diameter. The bank, built of earth and stone, survives to a width of roughly 2.2 metres, though its internal height is now only around 0.2 metres above the enclosed ground surface. Where it is best preserved, on the north-north-east to east arc and at the west-north-west, the inner face of the bank was carefully revetted with rubble stonework, giving it a vertical profile on the inside. This is a constructional detail that required deliberate effort and points to a structure that was built to present a reasonably imposing interior wall face. The fosse, where it exists, runs from the north-west to the south-east arc, is 4.4 metres wide and reaches about a metre in depth. A 2-metre-wide break in the bank on the north-east side, with a causeway bridging the ditch, marks the original entrance. A dry-stone wall running north-east to south-west also crosses the fosse, connecting to the outer face of the bank, though its relationship to the original construction or to later use of the site is not recorded.