The rock itself is fractured and prominent, rising to about 85 centimetres at its highest point, and its decorated face looks not outward toward the view but inward to the northwest. That decorated surface measures roughly 1.45 metres by 60 centimetres, and what it holds is very weathered and faint, the kind of thing that rewards patience and a low afternoon light rather than a casual inspection.

The motifs belong to the tradition of prehistoric rock art, a practice found across Atlantic Europe in which people worked abstract marks into exposed stone surfaces, most likely during the Neolithic or Bronze Age. At Coomasaharn the repertoire is modest but legible on close examination: two cupmarks, each about 3 centimetres across and 4 millimetres deep, sit slightly off-centre to the south of the panel. Cupmarks are among the most common and least understood elements of rock art, simple circular depressions struck or ground into the stone whose original significance remains a matter of debate. To the west there is a small triangular mark, roughly 5 by 4 centimetres, also shallow. The bulk of the decorated surface, however, is covered by multiple pickmarks of varying sizes, between 5 and 10 millimetres in diameter, distributed without any discernible pattern. One further feature runs almost the full width of the eastern side of the panel, a linear groove 78 centimetres long and up to 7 millimetres deep, oriented northwest to southeast. Its age is uncertain, and it may be a modern addition rather than part of the original composition. Two deep fissures to the north of the outcrop have collected enough soil for grass to take hold within them, giving the rock a slightly divided, settled appearance that contrasts with the deliberate, if enigmatic, marks on its face.