The rock itself is modest, no more than two metres long and less than a metre high, its decorated surface angled to the south-east. What covers that surface is weathered, faint in places, and partly submerged: a pool of water and sphagnum moss at the southern end likely conceals at least one further motif, meaning the full composition may never be seen in its entirety.

The carvings belong to the tradition of prehistoric rock art found widely across Atlantic Europe, and the vocabulary here is typical of the form. Cup-and-ring marks, the most common motif type, consist of a small circular depression, the cupmark, surrounded by one or more incised concentric rings, and their meaning remains genuinely unknown. At Coomasaharn, the decorated surface holds several variations: a partial cup-and-two-ring motif to the south-west, its outer ring left deliberately open to the north; a large oval cup-and-one-ring motif with a central cupmark eight centimetres across; a right-angled linear groove that terminates in a smaller cup-and-ring; and a meandering groove that branches and fades toward the north-west. Two large cupmarks to the south are accompanied by a triangle of three smaller ones between them. What makes the site particularly interesting is the evidence that the rock surface may have been deliberately prepared before the main motifs were applied: pickmarks spread across the stone in a way that suggests groundwork rather than decoration, a detail also observed at a closely related site located approximately 5.4 metres to the south.

The outcrop sits above the Behy River valley, and the surrounding blanket bog means the approach is rough underfoot. The decorated surface faces south-east, which affects how light falls across the shallow marks; low raking light, early or late in the day, tends to make weathered rock art more legible, bringing out the faint grooves and rings that flat midday light can flatten entirely.