What gives it away is a subtle undulation in the ground near the neck of the headland, the narrow isthmus where the land pinches before opening out to sea. There, an earthen bank and outer fosse, a defensive ditch dug just beyond the bank to slow or deter approach, survive in degraded form close to the southern cliff edge. A modern field boundary runs parallel to these earlier features, almost as if the landscape instinctively kept repeating the same line.

A promontory fort is one of the simpler but more resilient ideas in early Irish defensive architecture: take a headland naturally protected on most sides by sea or cliff, cut it off at the landward neck with a bank and ditch, and you have an enclosure requiring very little effort to defend. The headland at Doon fits this template loosely, though the evidence is cautious. Described in 2002 by Casey as a possible rather than definite example, the southwest-facing headland measures around 120 metres in length, with vertical cliffs dropping to a shingle shore below and no accessible route to the sea from within. No internal features are visible, so whatever activity once took place on the plateau has left no obvious trace above ground. At the southwestern point, a natural sink hole adds a further oddity to the site's character. The headland is now in active use as a quarry, which complicates any reading of what might once have been there and makes the survival of even these slight earthwork traces something worth noting.