This is a souterrain, a type of man-made underground structure common across early Christian Ireland, typically built from drystone without any mortar and used variously for storage, refuge, or both. What makes the Lissard example quietly remarkable is how intact its layout remains: roughly L-shaped in plan, it bends through the earth in two chambers connected by a low constriction known as a creep, a deliberately narrow passage that would have slowed down any unwanted visitor considerably.

The first chamber, entered through an opening at its eastern end, runs east to west and stretches nine metres in length. At its north-western corner, the creep leads through to a second, shorter chamber of four and a half metres, oriented north-east to south-west. The whole structure sits within a ringfort, the kind of enclosed farmstead that dots the Irish countryside in the thousands and dates broadly to the early medieval period, roughly 500 to 1000 AD. Souterrains of this kind were typically constructed at the same time as the enclosing fort, dug into the ground and lined with carefully laid drystone walls and roof slabs. That both the ringfort and its souterrain survive together at Lissard gives the site a completeness that is less common than it might appear.