It is domed, roughly 2.4 metres long and 1.4 metres high, and its upper surface is carved with a cross-shaped groove and six shallow cup marks, those small circular depressions, typically a few centimetres deep and wide, that appear on prehistoric stones across Ireland and Britain, and whose precise purpose remains genuinely unresolved. The boulder is known locally as the Piper Stone, and the stone circle nearby shares the same name on the old Ordnance Survey maps, a pairing that suggests the two were always understood to belong to each other, even if no one now knows exactly how.

The cross carved into the boulder's upper surface is not a simple scratching. One groove runs roughly east to west, tracing a vein of quartz in the granite, as though whoever cut it was following something already latent in the stone. The second groove runs north to south, and the two intersect to divide the boulder's top into four quadrants. The carving is uneven in a telling way: north of the intersection, the groove is well-defined; south of it, it becomes very shallow and trails down the side of the boulder, breaking the pattern observed everywhere else, where the grooves stay confined to the upper surface. The six cup marks are distributed across three of the four quadrants formed by the cross, with three in the north-west, one in the north-east, two in the south-east, and none at all in the south-west. Whether that asymmetry was deliberate or simply the result of interrupted or unfinished work is not known.

The boulder sits within a wider landscape that repays slow attention. The adjacent stone circle is the more immediately legible monument, but the Piper Stone, with its carved quartz-following groove and its lopsided scattering of cup marks, raises questions the circle does not. It is a protected National Monument under state guardianship, which means access is generally possible, though the ground around Athgreany can be soft, and the markings on the boulder's surface are shallow enough that low, raking light, early morning or late afternoon, makes them far easier to read.