The detail is easy to miss: it sits on the interior side of the fort's outer bank, close enough to the cliff that the ground itself seems to drop away almost immediately beyond it. What makes the spot quietly arresting is precisely that ambiguity, the stonework suggests a deliberate construction, but whether it served as a way in or a way out remains an open question.

The souterrain is associated with a promontory fort at Doon, a type of defensive enclosure that uses natural cliff edges as part of its boundary, supplementing the sheer drop with artificial banks and ditches on the landward side. Here, the entrance to the fort's interior measures three metres wide at the inner bank, approached by a causeway roughly eleven metres wide and more than twenty-five metres long, a substantial piece of landscape engineering that suggests the site was once a place of some significance. The survey of north Kerry carried out by C. Toal and published in 1995 recorded these dimensions and noted the possible souterrain opening, placing the site within a broader pattern of defended coastal positions across the region.