Whether the original wall ever continued across or beyond that edge is unknown; centuries of erosion have taken the answer with them. This ambiguity is part of what makes the fort so arresting. It is not a ruin in the conventional sense but a structure whose full geometry has been swallowed by the sea, leaving scholars to work with what remains of a site occupied, modified, and eventually abandoned across roughly two thousand years.

Excavations carried out between 1992 and 1995 as part of the Discovery Programme's Western Stone Forts Project, directed by Claire Cotter, established that the earliest activity here dates to around 1000 BC, in the Late Bronze Age, and that the site was finally abandoned around AD 1000. In its original form the fort was a hillfort, roughly D-shaped and enclosing approximately 5.8 hectares, defined by three concentric drystone limestone walls. The inner enclosure occupied the highest point of the plateau; unusually for Irish hillforts of this type, the middle enclosure did not wrap around it but was conjoined, extending eastward from it rather than surrounding it. The most dramatic transformation came in the early medieval period, when the fort was substantially remodelled. The inner enclosure wall was thickened and heightened to a maximum of 4.9 metres, with internal terraces and steps built into the fabric of the masonry. Outside the middle enclosure, builders added a freestanding curved wall 63 metres long and, beyond that, a chevaux-de-frise, a defensive field of densely packed upright limestone pillars covering 6,500 square metres. A chevaux-de-frise is an obstacle designed to break up and slow an advancing force; this one, the largest surviving example in Europe, consists of jagged pillars up to 1.75 metres high, many weighing close to half a tonne, levered from the surrounding limestone pavement and wedged upright with rubble. There is only one way through it: a narrow walled avenue on the north-east side. This remodelling effectively contracted the defended area, shifting the fort's character from a broad hillfort to something closer to a cliff-edge citadel, with strong affinities to high-status stone cashels elsewhere in the west of Ireland, including Dún Eochla on the same island and Cahercommaun in County Clare. The five external buttresses visible today on the inner enclosure wall are a later addition, added during 19th-century conservation works.

The approach to the fort from the village of Cill Mhuirbhigh involves a walk uphill across the limestone plateau, and the chevaux-de-frise is encountered before the walls themselves. Seen up close, the field of tilted and weathered pillars gives a clearer sense of the medieval builder's intention than any description can. The walled avenue channelling visitors through it to the north-east entrance is the same corridor that any attacker would have had to use, under whatever defensive scrutiny the terraced walls above afforded.