What makes it quietly arresting is not its scale but its deliberate incompleteness. Across the right-hand half of its surface, someone in deep prehistory carved seven small cup-marks, those shallow, circular depressions found on prehistoric stones across Ireland and Britain, whose purpose remains genuinely unresolved. The left-hand half was left entirely plain. Whether that contrast was meaningful or incidental is a question nobody can answer, but it is hard to look at the stone and not feel the intention behind it.

The boulder came to light during excavations carried out in 1945, led by archaeologists Ó Ríordáin and De Valera, whose findings were published between 1951 and 1953. It was found lying loose on the southern side of the wedge-tomb's cairn, in the space between the kerb stones and the inner revetment wall. A wedge-tomb is a type of megalithic burial monument common in Ireland, typically consisting of a roofed gallery that narrows towards one end, set within a cairn of smaller stones; the Ballyedmonduff example sits within that broader tradition. The cup-marks themselves are modest in size, ranging from 1.5 to 3 centimetres in diameter, arranged in a loose pattern: two well-spaced marks near the centre of the boulder, then a row of three stepping off to one side, and finally a pair positioned above and in line with the last of that row.

The site now sits in a forest clearing, the trees having grown up after the 1945 excavation, and they have entirely closed off what were once open views south and south-east across the Dublin landscape. The approach is on the lower slopes of Two Rock Mountain, and the forest setting means you need to know roughly where you are going before you arrive; the views that once made this a landmark in the terrain are simply gone. The boulder itself is embedded in the cairn material of the tomb, so what you are looking at is both the rock art and its original archaeological context, more or less as it was found. The cup-marks are small and can be difficult to read in flat midday light; an overcast day, or a visit in the lower sun of morning or late afternoon, will make the shallow carvings considerably easier to see.