Known locally as Dún Dúcathair, the Black Fort occupies a sheer headland where the limestone pavement drops away to the sea on three sides, leaving only the landward approach as a point of vulnerability. That vulnerability was taken seriously by whoever built the place. Stretching roughly 55 metres outward from the rampart is a dense field of chevaux-de-frise, upright stones set into the ground at close intervals to break the momentum of an attacking force, making any approach on foot slow, painful, and conspicuous. It is one of the most extensive such defensive fields associated with any Irish prehistoric fort.

The great drystone rampart that seals off the headland to the north-east runs about 73 metres long, stands nearly four metres high, and measures over seven metres wide at its base. A structure of that mass belongs to the Iron Age tradition of Atlantic stone forts, though the faces visible today owe something to nineteenth-century hands. Both the inner face, which carries three terraces connected by steps, and the buttressed outer face were substantially restored during that period. What the restorers could not recover was the western entrance. The antiquarian Thomas Johnson Westropp, writing in 1914 and drawing on George Petrie's observations from 1821, recorded that the fort once had a perfect entrance to the west, but that a great cliff-fall had destroyed it before 1839. The cliff, in other words, had been eating the fort long before any modern visitor arrived. Within the surviving enclosure, two groups of house foundations occupy the limestone pavement, their outlines still legible among the boulders.

The fort is accessible on foot from the road network on Inis Mór, though the walk across exposed terrain means the conditions matter more than on sheltered sites. The chevaux-de-frise stones require careful footing, and the cliff edge beyond the rampart is unguarded. The interior gives a clear sense of the original ground surface, bare limestone pavement scattered with glacial boulders, which makes the physical scale of the undertaking, building something this substantial in this material on this headland, quietly difficult to account for.