A souterrain, to use the proper term, is a man-made underground structure, typically stone-lined, associated with early medieval settlement in Ireland and thought to have served as storage space, a refuge, or both. The one at Cush is modest in scale but precise enough in its construction to reward close attention to the numbers alone.

The excavation was carried out between 1934 and 1935 by Seán P. Ó Ríordáin, whose published account from 1940 remains the primary record of what was found. The souterrain sat within what Ó Ríordáin designated Ringfort 10, a circular enclosure of the kind that dots the Irish countryside in considerable numbers, usually dating to the early medieval period. The stone-built section of the passage measured roughly 6.7 metres in length, though when the approach, a clay-floored entry where the ground was deliberately sloped down to meet the floor level, was included, the total ran to approximately 11 metres. The walls ran parallel to one another but were not straight, giving the passage a slightly irregular course. The gap between them came in at just under 1.2 metres, and the best-preserved sections stood a little over 1.2 metres high. That is enough headroom to move through in a crouch, but not comfortably upright.

Cush is not a site with an open visitor centre or marked trail, and the souterrain itself was examined during excavation rather than left accessible in any formal sense. Anyone with a serious interest in the structural details will find Ó Ríordáin's 1940 publication the most useful companion, since it includes plans of the feature. The ringfort complex at Cush was significant enough to attract sustained archaeological attention, and the site reference LI048-034007 will help locate it within the national monuments record. The surrounding landscape in this part of Limerick is quiet and agricultural, and the earthworks, where visible at surface level, tend to read more clearly from above than on the ground, which is why aerial imagery has become a useful supplement to the original excavation plans.