A promontory fort is exactly what the name suggests: a headland cut off from the mainland by an earthwork, turning natural geography into a defensive perimeter. What makes Dooncarton quietly interesting is not just the fort itself but the layering of occupation within it, from its original construction down to what may be the domestic ruins of a seventeenth-century family, all compressed onto a subrectangular headland measuring roughly 58 metres north to south and 28 metres across.

The neck of the headland is defended by a stone-faced earthen bank, still averaging 1.7 metres above the outer field level and rising to 5 metres above the bottom of the ditch beyond it. That ditch is steep-sided and V-shaped in section, nearly 20 metres wide at the top and close to 3 metres deep, with stone revetting still visible along both faces. A 2-metre causeway crosses it near the western cliff edge, forming the entrance route onto the headland. Inside, the interior slopes gently northward and holds the remains of up to five rectangular stone buildings, their walls now reduced to low, grass-covered rubble seldom exceeding a metre in height. The largest, measuring 16 metres by 10 metres, sits along the eastern cliff edge with a narrow doorway in its western wall; two further buildings of similar proportions occupy the western edge and the northern end respectively, all aligned along the axis of the headland. The antiquarian T. J. Westropp visited and described the site in 1912, noting features, including a wall of large blocks and a porter's lodge beside the bank, that are no longer visible on the ground. He also proposed that the internal buildings might represent the houses of the Cormick family, occupants of the site in the seventeenth century. Erosion since Westropp's visit has taken its toll, particularly in the southwest corner. A small circular hut, 5 metres in diameter, sits a further 15 metres outside the main enclosure along the eastern cliff edge, partly cut by a modern wire fence. The headland immediately to the northeast carries the remains of a separate promontory fort, making this stretch of poor-quality coastal pasture an unusually concentrated area of this type of monument.