It would be easy to walk past without a second glance. The stone measures less than a metre across, its decorated face is roughly the size of a sheet of paper, and the marks carved into it are so worn and faint that tracing them requires patience and a good raking light. What makes it worth looking for at all is what those faint marks represent: a cup-and-ring motif, one of the oldest known forms of decorative or symbolic carving in the prehistoric world.

Cup-and-ring marks are a recurring feature of the Irish and broader Atlantic prehistoric landscape. They consist, at their simplest, of a circular depression, the cupmark, surrounded by one or more concentric incised rings. No one has definitively explained what they meant to the people who made them, but they appear widely across Ireland, Britain, and parts of continental Europe, generally attributed to the Neolithic or Bronze Age. At Coomasaharn, the motif is modest even by the standards of the form: a single cupmark roughly five centimetres across and four millimetres deep, encircled by one ring about a centimetre wide. The whole composition sits in the southern corner of a triangular patch of sandstone at around 148 metres above sea level, on a northeast-facing slope that looks out over the Behy River valley. Adjacent to the carving, toward the northeast, there is a cluster of what may be pickmarks, though their relationship to the main motif is uncertain. The stone itself is slightly rough and fractured, and the combination of the rock's condition and centuries of weathering has left the carving almost at the threshold of visibility.