What they had stumbled upon was a souterrain, a type of underground stone-built passage or chamber associated with early medieval settlement in Ireland, and it had been roofed with nine large slabs, six of which turned out to bear ogham inscriptions. Ogham is an early medieval Irish script in which letters are represented by a series of notches and strokes cut along the edge of a stone, typically recording a person's name and ancestry. Along with the inscribed roofing slabs, a seventh stone was found standing upright inside the passage, apparently propping up a larger stone that had cracked at some earlier point. Bones and skulls, some reportedly human, were also found inside.

The stone now standing near the roadway at Coolmagort is one of six that had been laid flat as roofing material. It measures 0.86 metres high, up to 0.29 metres wide, and 0.10 metres thick, and was used specifically as a packing stone above two of the other inscribed slabs. It had already been damaged before it was ever pressed into structural service: the top is broken and the edge was chipped off in antiquity. What survives of the inscription was read by the scholar R. A. S. Macalister in 1945 as MAQI-DECEDA MAQ[I, a partial formula common in ogham texts, meaning something like "son of Deceda, son of...", with the remainder lost. In 1940, the Office of Public Works removed all seven stones from the souterrain and erected them near the road. The underground structure was then filled in, and no surface trace of it remains.

The stones stand close to a public roadway, which makes them straightforwardly accessible. The site itself carries a preservation order under the National Monuments Acts, and the stone has since been digitally recorded as part of the Ogham in 3D project run by the School of Celtic Studies at the Dublin Institute for Advanced Studies, which has produced detailed three-dimensional scans of ogham stones across Ireland. The partial inscription, the visible damage inflicted long before the souterrain was even built, and the question of whose bones were found alongside the stones make this a site where the layers of reuse and disturbance run unusually deep.