The stone known as Rathkenny I was not set upright as a monument or left in open ground; it was built into the roof of a souterrain, the kind of underground stone-lined passage and chamber system that was commonly constructed beneath Irish ringforts for storage or refuge. Specifically, it acts as a lintel, positioned centrally over the entrance chamber, its ogham inscription facing into the dark.

The ringfort itself is called Lismore, or An Lios Mór, and it sits on rising ground in County Kerry with wide views in most directions. It is a multivallate rath, meaning it is enclosed not by a single bank and ditch but by three concentric banks and fosses, which marks it as a site of some status. Four ogham stones in total were found within its souterrain system, all apparently reused as structural elements. Ogham is an early Irish script, typically cut as a series of notches and scores along the edge or face of a stone, and was used primarily between roughly the fourth and seventh centuries. Rathkenny I measures approximately one metre in length and forty centimetres wide, though its full dimensions are uncertain because it remains embedded in the structure. The inscription, cut in broad clear scores, reads COMMAGGAGNI MU[COI] ISAMMNN, a formula typical of ogham memorial stones, recording a personal name and tribal or family affiliation. An opening made into one of the souterrain chambers in the late 1970s has left the drystone construction of the tunnels and walls clearly visible, and the stone has since been documented in detail as part of the Ogham in 3D project run by the School of Celtic Studies at the Dublin Institute for Advanced Studies.