Look closely, however, and it reveals itself as a rare example of a Rathdown-type graveslab, a category of decorated medieval funerary stone found almost exclusively in the coastal parishes south of Dublin, and whose precise origins and meaning remain only partially understood.

The slab measures 0.9 metres long and 0.55 metres wide. At its centre is a single large cup-mark, a shallow circular depression carved into the surface, from which five lines radiate outward toward one end of the stone. A single line runs from the same cup-mark in the opposite direction, and on either side of that line, five more lines are arranged in a herringbone pattern, each pair meeting at an angle like the spine and ribs of a fish. The design is precise and deliberate, though its symbolic intention is no longer certain. Ó hÉailidhe noted and documented the slab in 1957, and it was further recorded by Healy in 2009. Rathdown slabs as a group are thought to date from the early medieval or early Norman period, and they represent a localised tradition of grave-marking that sets this corner of Leinster apart from the rest of Ireland.

The slab is secured to the wall of the church recorded under the site reference DU026-050002-, which sits on the eastern slopes of Carrickgollogan, the hill that rises above Rathmichael in the foothills of the Dublin Mountains. The ruins are accessible and the stonework can be examined at close quarters. Because the slab is mounted on a wall rather than lying flat, the incised decoration reads reasonably well even in overcast light, though a low raking light in the morning hours can help bring the shallower lines into relief. For those who want to examine the decoration in detail before or after a visit, a three-dimensional digital model of the stone is available at skfb.ly/oHr9p.