A souterrain is an underground stone-lined passage or chamber, typically built during the early Christian period for storage or refuge. Using ancient inscribed memorial stones as roofing slabs was not unheard of in Ireland, though the scale here, fifteen stones repurposed together, makes Ballyknock a rather remarkable case of one era cannibalising the monuments of another.

The stone covered by this particular record is a substantial piece of gritstone, standing just over four and a half feet tall and roughly a foot and a half wide. Its inscription is pocked, meaning cut by a series of small hammer blows rather than a continuous incised groove, along the dexter edge, which in ogham convention is the right-hand edge when facing the stone. Interestingly, the final three notches of the inscription are duplicated on the adjacent edges near the top, a detail that scholars have taken into account when reading the text. Macalister, writing in 1945, and McManus, in 2004, both arrived at the same reading: BRANAN MAQI OQOLI, a formula meaning Branan, son of Oqol, the kind of genealogical statement that these stones were raised to preserve. The stones are no longer at the site where they were found. This particular example is now held on permanent display in the Stone Corridor at University College Cork, a long vaulted passage that houses one of the largest collections of ogham stones in the world and where the Ballyknock inscriptions can be read at close quarters.