A cashel is a ringfort enclosed by a dry-stone wall, a form of early medieval settlement once common across Ireland, and this one at Iskancullin contains at its northern centre a souterrain, the type of underground chamber or tunnel built from stone and used variously for storage, refuge, or ventilation. What makes this particular example quietly compelling is its state of suspended incompleteness: the entrance at the west-northwest is choked with rubble, the passage narrows as it runs eastward, and then it stops, blocked again, though it can still be traced for a further metre beyond that point as if it has simply been paused rather than finished.

The construction is straightforward but careful. The walls are of dry masonry, meaning no mortar, just stone fitted against stone, and the roof is formed from thin slabs laid flat as lintels. When the passage was inspected in 1997, six of those roof lintels were still in place, a detail first recorded by the antiquarian T. J. Westropp in 1901. The full length runs approximately 4.8 metres from west-northwest to east-southeast, with a maximum width of just over a metre and a height that barely reaches 1.15 metres at its tallest point. It is a tight, low space, built for function rather than comfort. Westropp was one of the most prolific recorders of Clare's ancient monuments in the late nineteenth and early twentieth centuries, and his notes on this site remain part of the small body of evidence that survives for it.