A souterrain is an underground stone-built passage or chamber, typically associated with Irish ringforts and used for storage, refuge, or both. The one at Cush, however, carries an oddity that sets it apart from its neighbours: a small opening near the floor of its southern compartment that was clearly neither an entrance nor a drain, but which nobody has fully explained.

The site was excavated between 1934 and 1935 by Seán P. Ó Ríordáin, whose published account from 1940 remains the principal record. The souterrain lay within a bivallate ringfort, meaning one enclosed by two concentric banks and ditches, and its two long stone-built compartments were connected by a narrow passage just over 0.6 metres in length, with a crawlway barely 0.5 metres wide and 0.6 metres high. The southern compartment measured roughly 3 metres in length and just over a metre across. At its north-eastern corner, stones had been deliberately omitted at floor level to create a small opening, only about 27 centimetres wide and 37 centimetres high, giving onto bedrock that had been cut back slightly but not enough to allow passage. Ó Ríordáin identified it as a ventilation vent, comparable to vents found in other souterrains, though those examples placed the vent near the roof rather than the floor. Crucially, the way the rock slopes upward from the floor outward rules out drainage as a function. The structure also appears to pre-date at least one of the burials found at the site, including a cremation burial associated with a cordoned urn, which lay directly above a reddish sand-and-clay layer that had already partially filled the souterrain.

The souterrain itself is no longer accessible as an open chamber; the excavation was a research exercise rather than a public opening, and what visitors encounter at Cush today is the wider ringfort landscape. The site is a scheduled monument and the earthworks of the fort remain visible. Those with an interest in the excavation findings are best served by consulting Ó Ríordáin's 1940 publication, which includes plans and photographs of the souterrain's layout. The detail worth holding in mind is that small floor-level vent: deliberate, carefully cut into the bedrock, and still without a wholly satisfying explanation.