A souterrain, the term for an underground stone-built tunnel or chamber typically associated with early medieval Irish settlements, this one is unusual in that it sits not beneath open ground but actually within the fabric of the cashel wall itself. A cashel is a stone-walled enclosure, roughly equivalent in function to an earthen ringfort, and the incorporation of a souterrain into its structure suggests careful, deliberate construction rather than opportunistic addition.

The site was first recorded in 1982, when investigators found a small mural chamber of roughly 1.5 metres square tucked into the north-western section of the cashel wall, roofed with flat slabs. It was inaccessible even then, and what could be seen suggested it was only a fragment of something larger, perhaps a collapsed passage or chamber extending further into the structure. By 1990, when the Archaeological Survey of Ballinrobe and District examined the site, a clearer picture had emerged, though not an encouraging one. The chamber recorded at that point measured 3.8 metres in length, 1.7 metres wide, and just 1.1 metres high, orientated north to south and largely infilled with debris. Entry was only possible through a gap created by displaced lintels, the large horizontal stones that once formed the roof, having shifted out of position over time. Lavelle's 1994 publication of the survey recorded these dimensions and noted the degraded state of access.