Ogham is an early medieval script, typically carved as a series of notches and strokes along the edge of a stone, and most commonly used to record personal names, often as memorial inscriptions. What survives here is modest: a lump roughly nine inches long and four inches square, bearing what the scholar R.A.S. Macalister, writing in 1945, read as the tail end of an M and a few further marks he described as "not so certainly intended as letters". He retrieved the fragment from the wall of the fort itself, where it had been built in as ordinary building material, its inscribed face pressed into rubble.

Macalister's work on Irish ogham stones was systematic and wide-ranging, and his record of this site touches on a second, related puzzle. He suggested that another ogham stone, catalogued separately, had come from the souterrain within Rathcanning, a souterrain being an underground stone-lined passage associated with ringforts, probably used for storage or refuge. But the earliest written account of that second stone tells a different story: it appears to have come not from Rathcanning's souterrain at all, but from a different souterrain lying to the south, closer to the farmyard where the stone was eventually found, reused as a building element. The two accounts do not fully reconcile, and the question of exactly where each stone originated remains unresolved. The fragment Macalister recovered from the fort wall is now held at the National Museum of Ireland in Dublin, removed from the Cork hillside where it sat, half-legible, for an unknown number of centuries.