This is a cashel, a type of ringfort built from stone rather than earth and timber, and it has been losing its shape for a very long time. By the time anyone bothered to write it down formally, the entrance had already vanished entirely.

When the archaeologist M. J. O'Kelly recorded the site in 1944, the stone rampart was still traceable but barely upright, reaching no more than two and a half feet at its highest point. He noted an overall diameter of roughly 98 feet, about 30 metres, and placed it clearly within the bank of the surrounding hill fort. Six decades later, a survey by Condit and Coyne in 2004 measured the circular site at 19 metres east to west, found no evidence of the original coursing of the stonework, and reported that the dished interior held no visible features whatsoever. No entrance. No internal structures. Just a bowl of ground ringed by a degraded bank of stone. The two surveys together give an oddly satisfying picture of slow, patient erasure.

Knonainy Hill has associations that stretch well beyond this single structure, and the cashel sits within a layered landscape rather than in isolation. The hill fort bank that O'Kelly referenced as surrounding context is itself a significant earthwork, so arriving here involves reading one monument against another. Access to sites of this kind on sloped agricultural ground can depend on the season and on land conditions underfoot, so stout footwear is advisable. Once on the slope, the cashel is easy to miss precisely because it is so reduced; what you are looking for is a subtle circular depression ringed by a low, irregular scatter of stone rather than anything that announces itself. The dished interior that Condit and Coyne noted is probably the most reliable indicator that you are standing in the right place.