A souterrain is a man-made underground structure, typically stone-lined, used in early medieval Ireland for storage or refuge, and the one at Lavallyconor is a reasonable example of how thoroughly these features can slip from view. What remains visible is a rubble-strewn corridor measuring more than 14.8 metres in total length, though the northeast end disappears entirely under a mound of collapsed material, leaving open the question of how much further it once ran.

The passage follows two axes: a longer arm of 9.7 metres oriented roughly west-northwest to east-southeast, and a shorter arm of 5.1 metres branching off to the northeast. Its width varies considerably along its length, from 1.3 metres at the narrower sections to as much as 3.9 metres at its broadest, which suggests either deliberate variation in construction or significant collapse and distortion over time. Broken stone slabs scattered nearby may once have served as roof lintels, the flat capstones that would have sealed the passage overhead. The souterrain sits within the cashel itself, a cashel being a roughly circular enclosure defined by a stone wall, typically associated with early Christian period settlement in the west of Ireland. McCaffrey noted the site in 1952, cataloguing it as part of a broader survey of the area's archaeological features, and the structural details recorded then remain the primary basis for understanding what survives.