The fort at Ballynagree is D-shaped, and the logic of that shape becomes clear once you understand the site's position: the straight side of the D is not a wall or a bank at all, but the cliff itself, dropping away to the east. Whoever built this enclosure recognised that nature had already done the hardest defensive work on one side.

The constructed elements fill in the rest of the perimeter. Two earthen banks run from the south-southeast around to the west, separated by a fosse, which is the ditch dug between them to increase the obstacle facing anyone approaching from below. A single, lower bank, just 0.6 metres high, continues the line from the west around to the north-northeast, where a narrow gap of about one metre marks the entrance. The southern approach, where the land slopes downward outside the enclosure, is the most heavily defended stretch, the banks here being incorporated into the hillside in a way that makes them appear more formidable from below. The interior is mostly level, though it tilts toward the cliff along the eastern margin. The overall dimensions, a straight side of 20 metres and a projection of 18 metres to the west, suggest a modest but carefully planned enclosure rather than a large communal stronghold. Earthworks of this kind, typically referred to as ring forts or raths, were a common form of rural settlement and enclosure in early medieval Ireland, though cliff-edge examples that substitute a natural drop for one full side of the boundary are less frequently encountered.