The decorated face measures roughly 4.2 metres by 2.1 metres, and across it prehistoric hands pecked out a grammar of circles and hollows: cup-and-ring motifs, in which a shallow circular depression is surrounded by one or more incised rings; fifteen simple cupmarks; a cup-and-two-rings; and three short straight grooves, two of them running parallel. What is quietly remarkable about the arrangement is its precision in one particular respect. The rock surface is crossed by natural fault lines, and the motifs avoid every one of them, suggesting that whoever made them worked carefully around the existing fractures rather than treating the stone as a blank surface.

Rock art of this kind is generally attributed to the Later Neolithic or Early Bronze Age, a broad window running roughly from 3500 to 1500 BC, though pinning down individual sites is difficult since the carvings contain no datable material. The Iveragh Peninsula has a notable concentration of such decorated outcrops, documented in the 1996 Cork University Press survey by A. O'Sullivan and J. Sheehan. The Derrygarrane boulder sits at around 191 metres above sea level on a west-facing slope, and its position is not accidental in feel, even if intention is impossible to prove. From the crest of the low hill on which it rests, the view opens south along the Blackwater river valley towards Kenmare Bay, with the Beara Peninsula visible beyond. To the north, Knocklomena itself fills the skyline. A standing stone lies approximately thirty metres to the southwest, suggesting the boulder was part of a wider, organised presence in this stretch of upland.

The decorated surface repays close attention. At the western end, a large oval motif encloses three cupmarks of graduated sizes. Centrally, a substantial sub-circular motif is defined on its eastern side partly by a natural fracture in the rock, its cup around twelve centimetres across. Clusters of pickmarks appear in several places, particularly near the western cup-and-ring pairs. The motifs are shallow, some only two or three millimetres deep, so low-raking light in the early morning or late afternoon makes them considerably easier to read than they would be at midday.