Ogham is an early medieval script in which letters are represented by groups of notches and strokes cut along a central line, typically the edge of a standing stone. What makes the Kilmartin example unusual is where it ended up: not upright in a field or built into a church wall, as so many ogham stones were repurposed, but laid flat as a lintel, a roofing slab, inside a souterrain. A souterrain is a man-made underground chamber or passage, usually constructed during the early medieval period and associated with nearby settlement sites, most likely used for cool storage or as a place of refuge. The inscription here runs along three sides of the stone, which measures roughly 1.7 metres in length, suggesting it was a substantial standing stone before it was reused in this subterranean context.

The reading of the inscription was made by R.A.S. Macalister, whose 1945 corpus of ogham stones across Ireland remains a foundational reference for the field. He transcribed the Kilmartin stone as UDDMENSA CELI NETTASLOGI, a formula broadly consistent with early Irish memorial inscriptions that record a person's name and ancestry. The stone, then, is almost certainly funerary or commemorative in origin, carved perhaps in the fifth or sixth century, before being incorporated into the souterrain at some later point. The passage itself has since become inaccessible, and the stone is presumed to remain in position where Macalister recorded it, hidden within the collapsed or sealed structure, still carrying its carved names in the dark.