The rock is modest in size, roughly 1.4 metres across and 0.7 metres at its highest point, and it sits only a little above the level of the path beside it. But the face of it, convex and inward-looking towards the trackway, is covered in prehistoric rock art: cup-and-ring marks, which are circular grooves carved concentrically around a shallow central hollow, along with meandering grooves that wind across the surface, one of them running almost the full length of the boulder. Cup-and-ring carvings of this kind are found across Atlantic Europe and are generally assigned a Neolithic or Bronze Age date, though their meaning, if they carried a single meaning, remains genuinely unknown. What makes the Coomasaharn boulder a little stranger is that someone, at some later and unrecorded point, added the incised letters M, Q and I to the same surface.

The full repertoire documented by researchers includes nine cup-and-rings, a cup-and-two-rings with a radial groove from which three meandering grooves extend, three cupmarks with radial grooves, and five plain cupmarks. A number of the ring motifs appear to be gapped, though whether this reflects an original feature of the carving or simply the accumulated effect of weathering is unclear. The boulder is described in O'Kelly's 1958 survey and again by Finlay in 1973, and was later included in A. O'Sullivan and J. Sheehan's archaeological survey of the Iveragh Peninsula, published by Cork University Press in 1996. By the time of more recent inspection, encroaching bog, surrounding vegetation, and lichen growth had begun to obscure much of what earlier researchers recorded. Of the three incised letters noted by O'Kelly and Finlay, only the letter I remained legible.

The boulder sits at around 185 metres above sea level, set into the southern edge of a bog trackway with a turf-spreading area immediately to the south. The decorated face looks inward towards the trackway rather than outward to the landscape, though from this spot there are long views across the Behy River valley to the north-east, reaching as far as Glenbeigh. The motifs themselves, formed by careful pickmarks a few millimetres deep, are still visible in the exposed section of the surface, though the full extent of the carving continues to disappear beneath the bog.