The lintelled doorway, a classic feature of early medieval underground stone passages known as souterrains, measures just 45 centimetres wide and 25 centimetres high, barely large enough to admit a person crawling flat. These underground chambers, built of stone and roofed with large flat slabs called lintels, were constructed throughout early medieval Ireland as places of refuge, storage, or concealment, typically within or beside the enclosures of ringforts and cashels.

The souterrain sits within Cahernahooan cashel, a stone-walled circular enclosure of the kind built by farming and pastoral communities in Ireland from roughly the early medieval period onwards. The underground passage extends at least two metres to the west-northwest from the entrance, though that measurement represents only a confirmed minimum. Hollows visible to the south and east of the entrance suggest the structure branches or continues in at least one further direction, meaning the full extent of what lies beneath the surface remains unknown. It is a common situation with souterrains, many of which were partially blocked or collapsed over centuries, leaving surface depressions as the only outward sign of what once lay below.

The entrance itself is the most accessible part of the site. The cashel that contains it sits in the townland of Ballycahill, and the souterrain opening can be seen within the north-western sector of that enclosure. The aperture is small enough that it reads almost as incidental until you understand what it leads into, a deliberate design feature in structures built for concealment rather than convenience.