What sets it apart is a shallow circular depression ground into its upper surface, a cupmark roughly seven centimetres across and eight millimetres deep, placed with apparent deliberateness close to the boulder's eastern edge. Cupmarks are among the oldest forms of human mark-making found in Ireland, carved by pecking or grinding stone against stone, though their purpose remains genuinely unclear to archaeologists. This one is accompanied by scattered pickmarks elsewhere on the same surface, traces of further working that hint at something more than accidental damage.

The boulder is built into the exterior southern wall of the fort, 3.7 metres southwest of the entrance, and there is reasonable grounds to think it was always there. Restoration work carried out on the fort during the 1980s was confined to the western side of the exterior wall, leaving this southern section largely untouched, which means the boulder may well be original fabric rather than a later insertion. That possibility gives the cupmark an added layer of interest: whoever built the fort either carved the mark themselves, or chose a pre-marked stone and laid it deliberately into the structure. A second rock art panel has been recorded just six metres away. Beyond the fence enclosing the fort, further marked stones appear in the surrounding landscape, a cupmarked boulder sits within a field clearance cairn twenty metres to the south, and a cup-and-ring marked boulder, where the cupmark is surrounded by one or more carved concentric rings, lies in field clearance about eighty metres to the southwest. The concentration suggests this small coastal area held some significance long before the fort itself was raised.