What makes this particular souterrain more than just another underground chamber is a single lintel stone carrying an ogham inscription, the ancient Irish script in which letters are rendered as notches and strokes along a central line. That inscribed stone sits at the innermost end of the second chamber, an unlikely place for an inscription that would, in most circumstances, be meant to be read.

A souterrain is an underground stone-built passage or set of chambers, typically associated with early medieval ringforts in Ireland, and thought to have served variously as storage, refuge, or means of escape. The Kilmartin example sits just west of the centre of its ringfort and was examined twice in the twentieth century: by Hartnett in 1939 and by Coleman in 1947. Coleman's account describes two rectangular chambers roofed with flat lintels and connected by a short earth-cut creepway, a narrow low passage just four feet long. The first chamber ran north to south, measuring nine feet long, three feet wide, and four and a half feet high; Coleman entered it through the collapsed southern end. The second chamber ran at right angles to the first, and had already suffered considerable collapse at its western end by the time it was inspected. It was on the innermost lintel of this second chamber that the ogham stone was recorded. Using a carved roof-stone as a writing surface is unusual; ogham inscriptions are far more commonly found on standalone pillars or on stones reused in later constructions, and their presence deep inside a souterrain raises questions that have not been straightforwardly answered.

The souterrain is no longer accessible as an intact structure. The collapsed ground is now the primary indicator of where it lies, and the ogham-inscribed lintel remains in situ underground rather than in any display collection. Anyone visiting the site would be looking at a grassy depression within a ringfort earthwork rather than any visible architecture, which is in itself a reminder of how much of early medieval Ireland persists not as monument but as subtle disturbance in the landscape.