Souterrains, from the French for "underground passage", are dry-stone-built underground chambers associated with early medieval ringforts across Ireland, thought to have served as refuges, storage spaces, or both. This one adds a particular layer of curiosity: it is L-shaped, and what can be seen of it may only be part of what was originally built.

The souterrain sits in the north-east sector of a rath, the term used for a circular earthen ringfort of the early medieval period. The first passage runs on a north-west to south-east axis, measuring more than 4.4 metres in length and 1.6 metres across, with entry gained at its north-western end. A linear depression in the ground, still showing traces of dry-stone walling along its edges, suggests the passage once continued further to the north-west before whatever event or process caused it to collapse or become obscured. The second passage branches off from the south-eastern end and appears to run roughly east to west, though it is currently inaccessible. Together, the two corridors form the characteristic bent plan that gives the structure its L-shaped layout, a design sometimes thought to have aided concealment or slowed unwanted entry.