The markings are easy to miss entirely, and that difficulty is part of what makes them interesting. Prehistoric rock art in Ireland typically takes the form of cup marks, carved rings, or more elaborate abstract motifs pecked or ground into stone surfaces; what survives at Coolycasey is modest even by those standards, a short groove and a row of small indentations covering an area of roughly 30 by 17 centimetres. Whether these were made by human hands centuries or millennia ago, or are simply the product of weathering, remains an open question, though the straight groove at the boulder's western edge has been assessed as unlikely to be natural.

The boulder was reported by Micheál Mac Gearailt. It is an irregular, roughly set stone, possibly not a true bedrock outcrop but a placed or settled boulder, measuring just under a metre in its longest dimension and tapering from a height of 0.63 metres at the north end to around 0.2 metres at the south. The rock is sandstone with several quartz inclusions, some of them showing a green mottled surface coating. The upper face rises to a sharp northern edge, and it is along that top western edge that the groove runs east to west, 9.5 centimetres long and 2 centimetres wide. A short distance to the east of the groove, a series of indentations, each around 12 centimetres across, runs in a row. Much of the boulder is obscured by moss on its lower sections and lichen across the top, including over the marked area itself. A second boulder of similar geology sits 7 metres to the west, but carries no visible markings.

The hill sits in a pocket of unreclaimed ground surrounded by improved pasture, and commands a wide view across the Fergus estuary to the north-west and south-west, with low undulating hills visible to the north and east. The cleared stone scattered across the hilltop suggests the area has been worked or managed at some point, though the boulder itself was likely left in place simply because it was too awkward to move. The markings, if they are indeed anthropogenic, reward patience; the row of indentations in particular becomes far more legible in oblique light than under direct overhead illumination.