It belongs to a souterrain, an underground stone-lined tunnel or chamber built in early medieval Ireland, typically for storage or refuge, and it lies sealed inside a cashel, a type of circular stone-walled enclosure that served as a fortified farmstead. The blocked entrance points to a passage running roughly east-north-east to south-west, and that is, more or less, all that can currently be known from above ground.

The souterrain sits within the northern sector of the cashel's interior, a placement that is not unusual. Souterrains were commonly built close to the inner face of a cashel wall, making use of the shelter and structural support the enclosure provided. The Ballynastaig example was recorded by McCaffrey in 1952, noted at the time as blocked and inaccessible. The visible lintel, a flat stone spanning the top of the passage opening, is the kind of detail that quietly rewards a careful eye; it indicates the scale and construction method of whatever lies beneath, even if the interior remains closed off. Whether the blocking was deliberate and historic, or the result of later collapse and accumulation, the record does not say.

Because the souterrain is inaccessible and the site itself is on private agricultural land typical of this part of Galway, there is little a visitor could do beyond locating the cashel enclosure and noting the lintel from the surface. The cashel itself, recorded separately, is the more visible feature.