The fifteenth simply vanished. Ogham is an early medieval Irish script in which letters are represented by groups of notches and strokes cut along the edge of a stone, and the souterrain in question, an underground stone-lined passage typically associated with a nearby ringfort, proved to be one of the most concentrated finds of such stones anywhere in Ireland. This particular stone, a sandstone slab measuring just over a metre in height, was found loose inside that passage, and its inscription was read by different scholars as beginning MAILAGURO MAQ[I...], suggesting a personal name and a patronymic formula common to ogham texts.

When thirteen of the group were removed to University College Cork and one to Lismore Castle in the early twentieth century, the farmer at Ballyknock chose to keep this stone back. That decision, recorded by Power in 1932, was unremarkable enough at the time. What followed was less so. When the scholar R. A. S. Macalister returned to examine the stone before 1945, having previously read its inscription, he found the farm deserted and the stone gone entirely. No explanation was recorded. The inscription he and O'Kelly had attempted to decode, with its partially legible name and lineage, remains unresolved, its latter portion lost along with the stone itself.

The ringfort and souterrain at Ballyknock remain the fixed point in this story, the place where fifteen carved stones were gathered, presumably over centuries, before their scattered dispersal in the modern era. What the farmer intended for the stone, or what became of it, is not known.