The result is not monumental in scale, but the precision is deliberate: two linear grooves meeting at a right angle, and a scattered arrangement of cupmarks, small circular depressions pecked into the rock, grouped around those grooves in a pattern that feels considered rather than accidental. Cupmarks are among the most common yet least understood motifs in prehistoric rock art; they appear across Ireland, Britain, and much of Atlantic Europe, usually without written context, their meaning long since dissolved into the past.

The outcrop at Coomasaharn sits at roughly 148 metres above sea level on a north-east-facing slope, looking out over the Behy River valley to the north, in what is now cutaway bog, landscape from which peat has been removed over generations. The rock itself is sandstone, slightly rough and fractured, measuring just under three metres along its longest axis. The decorated portion, concentrated at the flatter western end, is smaller still, less than a metre across. Within that compact space, the carver cut one groove running northeast to southwest, nearly half a metre long, and a second branching from it at a right angle toward the southeast. Two larger cupmarks sit to the northwest of the first groove; four smaller ones are arranged in pairs to its southeast, two on either side of the second groove. A natural fissure runs across the rock independent of all this, a reminder that prehistoric carvers often worked with stone that already had its own character, sometimes incorporating such features, sometimes simply working around them.