The stone-built chamber sits visible on the south side of the road, with several large flat slabs scattered nearby that were almost certainly once its roof lintels. On the north side, a second chamber survives, roughly a metre high and a metre and a half long, curving eastward before the passage meets a wall of collapsed material.

A souterrain is an underground stone-lined passage or series of chambers, typically associated with Early Medieval ringforts and used variously for storage, refuge, or the cool preservation of dairy produce. Ringforts, the enclosed farmstead settlements of early Christian Ireland, were built in their thousands across the country, and it was not unusual for them to contain such subterranean additions. The ringfort at Lisnacask sits on a slight rise in gently undulating grassland, a typical positioning that would have offered a modest but useful vantage over the surrounding land. The souterrain lay hidden at the monument's centre until the roadway cut through, dividing both the fort and the underground structure in two. The monument was placed on the Register of Historic Monuments, as recorded in Iris Oifigiúil on 11 November 1983, a formal protection that arrived some time after the road had already done its damage.