Ogham is an early medieval script in which letters are represented by a series of notches and strokes cut along the edge of a stone, and it appears most commonly on standing stones rather than on the roofing slabs of subterranean passages. That someone chose to inscribe, or re-use, a marked stone here adds a layer of meaning that has never been fully unpicked.

The chamber itself is a souterrain, a type of stone-built underground structure associated with early medieval ringforts across Ireland. Souterrains were typically used for storage or as places of refuge, and this one sits within what was once a ringfort, now levelled, at Lackabane. The passage is rectangular, roughly three and a half metres long, just over a metre wide, and one and a half metres high, oriented roughly north-northeast to south-southwest. Its walls begin with a course of large flat slabs, each around a metre long and forty centimetres wide, stacked to half a metre in height, above which the stonework continues to the lintels that form the roof. J. P. McCarthy examined the site in the early 1980s and recorded both the chamber's dimensions and the presence of the ogham-marked lintel, a detail that lifts the structure well above the ordinary run of field monuments.