It was only turf cutting that brought it back into the light, revealing on its flat upper surface a carved motif that had been waiting, unseen, for however long the peat had been accumulating above it. The carving is small relative to the boulder itself, occupying a narrow decorated surface of roughly half a metre by twenty centimetres on a stone measuring over two metres in length. What it shows is a penannular cup-and-ring, the kind of prehistoric rock art found across Atlantic Europe, in which a small central cup-mark is surrounded by a carved ring, here left deliberately open at one point, with a curvilinear groove running outward from the cup to the exterior of the motif. A second, isolated cup-mark sits about thirty centimetres to the south, with possible faint traces of a ring around it.

Cup-and-ring marks are among the most widespread and least understood forms of prehistoric art in the British Isles and Ireland, carved into rock surfaces during the Neolithic and Bronze Age periods, though their precise purpose remains a matter of scholarly debate. The motif at An Gabhlán Ard is notably weathered, the incised lines only two to four millimetres deep, suggesting considerable age and exposure even before the bog covered it. The site was recorded by J. Cuppage in the Corca Dhuibhne archaeological survey published in 1986, which documented the boulder shortly after it was uncovered. The landscape around it is cutaway bog and mountain heath, with open views down toward the Owenalondrig River valley and, at certain angles, a narrow glimpse of the sea at Acres Point and Trá Chathail to the south-west, the mountains closing in from the north and east.