Ogham is an early medieval writing system in which letters are represented by groups of notches and lines cut along the edge or angle of a stone, and it was used primarily to record names and lineages. The Breastagh stone is among the more imposing examples: roughly rectangular in cross-section, with its top cut obliquely so that it tapers to a point on one side, it has inscriptions running along both the north-east and south-east angles.

When the stone came to scholarly attention in the 1870s, it was lying flat on the ground, and it appeared to be resting over what may have been a grave. At the southern end of that disturbed area, investigators identified a socket cavity, suggesting that this was the original position in which the stone had once been upright. The stone was re-erected shortly afterwards in the spot where it now stands. The inscriptions, though partially eroded and partly obscured by lichen, have been read on the right-hand angle as MAQ CORRBRI MAQ AMMLLỌNG[I]TT, translating roughly as "son of Coirpre, son of Amalgaid." The left angle preserves a more fragmentary text. Amalgaid is a name associated with early Connacht kingship, which lends the stone a degree of historical weight beyond its considerable physical presence. The stone's total length, including the buried portion, is around 3.6 metres. Approximately 300 metres to the north lies a wedge tomb, a type of megalithic burial monument predating the ogham tradition by several thousand years, giving the surrounding landscape a layered quality that stretches from the Neolithic into the early medieval period. The stone has since been examined as part of the Ogham in 3D project run by the School of Celtic Studies at the Dublin Institute for Advanced Studies, which uses digital scanning to capture inscriptions that are difficult to read with the naked eye.