What makes it unusual is partly what it is, and partly how little of it remains legible. The decorated area measures just 0.70 metres by 0.43 metres, a modest patch on an otherwise rough and fractured rock that stands two metres high and stretches more than five metres east to west.

The boulder is a glacial erratic, meaning it was carried here by ice and deposited far from its original geological context, long before anyone thought to carve into it. At some point in prehistory, someone worked the flat table-top surface of this displaced rock, cutting two simple cupmarks, each between four and five centimetres across, and three cup-and-ring motifs. Cup-and-ring marks are among the most widespread yet least understood forms of prehistoric rock art found across Atlantic Europe; they consist of a central hollow, the cup, surrounded by one or more incised concentric rings. The rings at Derrynablaha are approximately 13.5 centimetres in total diameter, with the carved channels only two millimetres deep. One of the three may have carried a second ring, though the weathering is severe enough that this cannot be confirmed with certainty. The motifs are confined to the south-western corner of the decorated surface, and all of them are very faint. The Kealduff River runs close to the north-east of the site, and the slope faces east, sitting at roughly 101 metres above sea level. Trees from the adjacent plantation are encroaching from both east and west, a slow pressure that does nothing to help the long-term visibility of the carvings.