That detail alone sets it apart. A cashel is a type of early Irish stone-walled enclosure, typically circular, used as a farmstead or defended homestead, and souterrains, underground passages constructed from large stone slabs, are a fairly common feature within them. They served various purposes, from food storage to escape routes to places of temporary concealment. Here, though, the tunnel's path is unusually deliberate: it threads beneath the full depth of the defences rather than simply dipping under a single wall.

Redington, writing in 1916, described the souterrain as passing under both ramparts to reach the exterior of the fort, and the structure is still traceable in the northern sector of the cashel, lying to the east of an internal wall. Three of the large flat roof lintels that would originally have covered and defined the passage remain in place; a fourth has been removed at some point, leaving a gap in the sequence. The passage itself runs roughly north-east to south-west and measures about 3.9 metres in length, though that figure likely reflects only the accessible or surviving portion rather than the full original extent implied by Redington's account. McCaffrey also recorded the site in 1952, confirming what remained visible above ground.

The interior of the passage is unfortunately inaccessible, having been used as a dump, so the stonework of the walls and any details of its construction cannot presently be examined from within. What can be seen are those three remaining lintels, sitting quietly in the grass of the cashel's northern half, marking the line of something that once passed invisibly beneath the feet of anyone standing at the rampart above.