The slab itself is only a fragment, the upper portion of what was originally a larger piece; the shaft and base of the cross are gone. What survives, measuring just 63 centimetres long and 37 centimetres wide, is nevertheless remarkably detailed, and it carries one of the more quietly poignant inscriptions in early Irish stone carving: OR DO, meaning "a prayer for", followed by a single letter, C, and then nothing.

The incised decoration on the slab is elaborate for its size. The cross is cut in five lines and enclosed within a rectangular frame, with semi-circular or D-shaped terminals, the ends of each arm of the cross, left plain and unadorned. The visual complexity is concentrated at the centre, where the arms meet: a circular panel filled with a key or fret pattern, itself divided into three sections, two of which contain pairs of opposed Z and S shapes, and the third a pair of angular spiral forms. This kind of geometric ornament, sometimes called interlace or knotwork in a looser sense, is characteristic of early medieval Irish ecclesiastical stonework. The inscription, written in half-uncial script, a rounded bookhand used by Irish scribes from roughly the sixth century onwards, runs along the upper right side of the slab. Higgins, writing in 1989, dated the piece to somewhere between the late eighth and the tenth century. Harney, reviewing the form in 2011, drew comparisons with similar cross-slabs at Clonmacnoise, tentatively dated to the late ninth or tenth century, and with examples from High Island off the Galway coast, where associated burials have been radiocarbon dated to the tenth and eleventh centuries.

The slab no longer sits in the Coolcashin graveyard where it was found, beside the remains of a medieval church in rolling grassland east of a small stream. It has been moved to the parochial house in Lisdowney, some distance away, where it is now kept. Whoever C was, commemorated in a prayer carved into sandstone over a thousand years ago, the stone that bears their initial has travelled considerably since.