The stone, inscribed with the name MOESAC in fine knife-cuts, was not standing upright in a field or propped against a churchyard wall when it was found. It had been repurposed as a structural component of an underground passage, serving as the entrance jamb, with its upper portion crushed and shattered by the weight of the lintel pressing down on it over the centuries.

Ogham is an early medieval script, most commonly encountered in Ireland and western Britain, in which letters are represented by groups of notches and strokes cut along a central stemline, typically running along the edge of a standing stone. A souterrain, meanwhile, is a man-made underground stone-lined passage or chamber, associated in Ireland with early medieval settlement sites and likely used for storage or refuge. The fact that an inscribed ogham stone was being used here as raw building material suggests the souterrain was constructed, or at least modified, at a point when the stone's original commemorative function had been forgotten or simply disregarded. The stone itself is modest in scale, roughly 0.97 metres long and made from soft shale or clay slate, and the inscription MOESAC almost certainly preserves a personal name, possibly that of an individual commemorated in the early centuries of the first millennium. Scholars including D. McManus, writing in 1997, have documented the reading of the inscription.

The stone is no longer at Underhill. It was removed and is now held in the National Museum of Ireland, where it joins a wider collection of ogham stones gathered from sites across the country, many of them similarly displaced from their original contexts long before anyone thought to record them properly.