At Ballyknock in County Cork, that is precisely what was found: a souterrain, the term for a stone-lined underground chamber or tunnel commonly built in early medieval Ireland, covered over by a remarkable collection of inscribed stones. One of those stones, a gritstone slab standing four feet tall and measuring roughly one foot three inches by six inches across, carries an inscription that is both legible and tantalisingly incomplete.

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, most often recording a personal name and ancestry. The Ballyknock stone is no exception in its general form, though a section of the inscription has been lost to damage. The chiselling is described as bold, suggesting the carver worked with confidence, but chipping over the centuries has opened a gap estimated to hold somewhere between eight and ten scores, meaning several letters are simply gone. What survives reads ACTO MAQI [....] MAGO, a formula following the standard ogham pattern of a name, then MAQI meaning "son of", then a father's name or lineage term, with the middle portion missing. The stone was read in this form by R. A. S. Macalister in 1945 and subsequently by Damian McManus in 2004, both arriving at the same partial reconstruction.

The stone is no longer at Ballyknock. It is held on permanent display in the Stone Corridor at University College Cork, where it sits among other ogham stones from across the region, allowing visitors to examine the inscription directly, lacuna and all.