Known as An Dún, the site consists of two conjoined oval enclosures, their north-western perimeters defined not by any man-made construction but by the sheer drop of the cliff-face. On every other side, a deep fosse, a rock-cut or dug defensive ditch, separates the two raised interior platforms from an outer earthen bank. The whole arrangement sits about 350 metres east of the mountain's summit, at 286 metres above sea level, fully exposed to whatever the Atlantic sends in from the north-west. It is the kind of place that makes you wonder about the priorities of whoever built it, and the kind of people who then apparently lived inside it.

The platforms themselves are substantial. The western one measures roughly 13 metres east to west and 7 metres north to south, and as recently as 1950, the writer Mason recorded two large circular clocháns, the corbelled dry-stone huts characteristic of early Irish settlement, still visible within it. Those are now reduced to spreads of collapsed stone. The eastern platform is larger, around 19 metres by 14 metres, and contains what may be the remains of a souterrain near its centre, an underground passage or chamber of the kind sometimes used for storage or refuge in early medieval Ireland, though the feature is too damaged for a firm identification. The defensive architecture, where it can still be read, involved drystone revetment, possibly in two tiers, lining the steep inner face of the fosse and raising the platforms some two to three metres above its base. A trackway running up from the village at the north-eastern end of the island passes immediately outside the outer bank; it was probably laid out to bring turf down from the mountain, which gives the fort an oddly domestic neighbour.

The site is genuinely ruinous and the detail difficult to interpret, but that difficulty is part of what makes it interesting. The most plausible original entrance appears to be a narrow 1.6-metre break on the south-eastern side of the western enclosure, where a corresponding gap in the inner defence lines up with it, suggesting a deliberate and carefully controlled point of access into what was, in its day, a well-engineered position. Sheep-shelters built by later islanders now occupy parts of the fosse and interior, a quiet illustration of how a promontory fort slowly becomes a farm fixture over the centuries.