A souterrain is an underground stone-lined passage, typically constructed during the early medieval period, used variously for storage, refuge, or both. What makes this one quietly compelling is the deliberate geometry of its route: rather than running in a straight line, it shifts direction three times, a design that would have slowed and disorientated any intruder moving through the dark.

The passage runs to approximately 7.5 metres in length and varies between 1.1 and 1.4 metres in width. Starting near the cashel wall on the west side, a cashel being a ringfort enclosed by a dry-stone wall, the first section runs roughly north-northwest to south-southeast for about 1.5 metres, before turning to run east-southeast for 2 metres, and finally northeast for a further 4 metres. That final stretch carries the heaviest damage, where lintels, the flat stones laid across the top of the passage to form its roof, are cracked and broken, leaving parts of the passage open to the sky. At the far end, a single large lintel marks the threshold into the beehive chamber, a corbelled circular space at least 2 metres in diameter, now largely inaccessible behind a tangle of blackthorn growth at the centre of the cashel's interior.