Known in Irish as Carraig an Aifrinn, the Mass Rock, it measures roughly 1.2 metres long and 0.75 metres high, and on its north face someone has carved a sunken cross, 16 centimetres across, enclosed within a circle. A rough flagstone to the east provided a seat, most likely for the officiating priest. It is an unassuming arrangement, and that was entirely the point.

Mass rocks are found across Ireland and belong to the era of the Penal Laws, the body of legislation that, from the late seventeenth century onward, severely restricted Catholic worship and effectively banned the public practice of the faith. Priests who continued to say Mass did so in the open air, at remote or discreet locations, with lookouts posted for approaching authorities. This particular site at Rathduff appears in the estate records of a local landowner, William Meade Smythe, whose papers preserve a vivid note on the place. The cross carved into the boulder's face, Smythe's record suggests, was cut to commemorate the fact that Mass had been said there, not merely to mark a sacred spot in the abstract. The same record mentions that in earlier generations, old men and women could still be seen praying at the rock. The curving field boundary visible on the 1837 Ordnance Survey six-inch map follows the quarried edge of the same outcrop, which means the landscape around the boulder has been shaped and reshaped over centuries even as the boulder itself remained.

The site occupies an elevated position with open views to the north and east, which would have made it practical for gatherings that needed advance warning of unwanted visitors. The carved cross remains visible on the boulder's north face, a small but deliberate mark, precise enough in its dimensions to suggest it was made with some care rather than haste.