A souterrain, for those unfamiliar with the term, is a man-made underground stone passage or chamber typically associated with early medieval Irish settlements, thought to have served for storage, refuge, or ventilation. What makes the one at Cush particularly interesting is its complexity: a T-shaped plan cut into solid rock, with a deliberately constricted entrance, a ventilation shaft, roofed and unroofed sections, evidence of timber structures, and a drainage system that carries water to the western side of the enclosing earthwork. It is not simply a hole in the ground but something closer to a piece of infrastructure, designed with clear intent.

The souterrain was excavated by Seán P. Ó Ríordáin between 1934 and 1935, with findings published in 1940. His account describes the approach from the centre of the fort, where six post-holes cut into the bedrock indicated a timber structure of some kind leading toward the underground passage itself. The entrance was deliberately narrowed to around two feet (roughly 0.6 metres) by a projecting wall and a well-shaped rectangular upright stone. Beyond this point a passage ran southwards for about twelve feet before meeting a longer cross-passage at a right angle, the floor of which was set a step lower. One arm of this cross-passage extended roughly eighteen feet and ended at a ventilation shaft built into the wall and opening onto the inner side of the ringfort's fosse to the south-east. The opposite arm ran further, turning at an obtuse angle, and was partly roofed before narrowing into a stone-lined drain; a layer of charcoal found above the unroofed portion of that drain suggested it had once been covered in timber. Several post-holes and wall fragments found near the central hearth indicated associated above-ground buildings, though no complete structure could be identified.

Cush is recorded in the national monuments record under the reference LI048-034028. As with many excavated sites, what is visible above ground today is unlikely to convey the underground arrangement described by Ó Ríordáin, so consulting his published plan alongside any visit gives a much clearer sense of the scale and geometry involved. The site sits within a wider complex of ringforts at Cush, a landscape that rewarded close attention long before the 1930s excavations and continues to do so for anyone willing to read the ground carefully.