It is the sealed entrance to a souterrain, an underground passage built from stone, of the kind used throughout early medieval Ireland as storage space, a place of refuge, or both. The passage runs east to west, though beyond its orientation and its existence, the structure keeps most of its details to itself.

Souterrains are typically associated with ringforts and cashels, and this one sits within a caher, the Irish term for a stone-walled circular enclosure of the early medieval period. The Iveragh Peninsula, where Cahereighterrush is located, is exceptionally rich in such monuments. A. O'Sullivan and J. Sheehan documented the site in their archaeological survey of South Kerry, published by Cork University Press in 1996, drawing on both fieldwork and local information. The blocked state of the souterrain entrance means the passage itself remains inaccessible and unexcavated, leaving open questions about its construction, its length, and what it may once have contained.