A souterrain is an underground stone-built passage or chamber, typically associated with early medieval settlement and used variously for refuge, storage, or concealment. This one sits beneath a circular clochán, the Irish term for a dry-stone beehive hut of the kind found across the Corca Dhuibhne landscape, and it has been recorded as inaccessible, meaning no one, at least in recent times, has been able to get inside it.

The site goes by the name Clochán Bhóthar an Trasnaigh, and what makes it more than a straightforwardly ruined hut is the company it keeps. Two stones found nearby had each been inscribed with three crosses, most of them featuring expanded terminals, a style in which the arms of the cross flare outward at their ends. This detail was recorded by R.A.S. Macalister in 1899, and Macalister was one of the foremost recorders of early Irish inscribed stonework, so the observation carries weight. Cross-inscribed stones of this type are generally associated with early Christian activity, sometimes marking burial grounds, sometimes serving as devotional objects at sites that have long since lost their original context. Whether these two stones belong to a larger pattern of religious use across this stretch of the peninsula is not clear, but their presence alongside a sealed, unreachable underground structure gives the whole assemblage an atmosphere of things only partially understood.