What sets it apart from its neighbours is not its size, though it measures roughly 2.6 metres by 2 metres, but what has been carved into its surface: a set of prehistoric marks, quietly legible after what may be thousands of years, cut into a sub-rectangular panel facing south-east toward the lake below.

The decoration is sparse, which only makes each motif easier to read. Three cupmarks, simple circular depressions pecked into the stone, are distributed across the panel, the largest about 6 centimetres across. More elaborate are the two cup-and-keyhole motifs placed centrally on the surface. A keyhole motif in Irish rock art typically consists of a cupmark enclosed within one or more penannular rings, that is, rings that are nearly but not quite closed, with a straight groove radiating outward like a tail. Here, the central example has a cupmark surrounded by two such rings and a long groove extending 70 centimetres toward the north-east edge of the stone. The second keyhole, positioned about 10 centimetres to the north, follows the same form: a cupmark enclosed by two penannular ring grooves, measuring roughly 35 by 32 centimetres overall. Several short linear grooves complete the composition. The motifs are described as largely incomplete, though whether that reflects original intent or millennia of weathering on a fractured sandstone surface is difficult to say. Rock art of this type is generally associated with the Later Neolithic and Early Bronze Age, a broad span running from around 4000 to 1500 BC, though dating individual sites with precision remains difficult.

The boulder sits in a pasture field on the Iveragh Peninsula, the same mountainous finger of land that carries the Ring of Kerry road around its southern edge. Coomasaharn itself is a corrie lake, formed in a natural hollow scooped out by glacial activity, and the steep south-east-facing slope that holds the carved stone would have given anyone standing there a clear view across the water. Whether that orientation was deliberate is one of the questions the stone cannot answer.