A souterrain, as the word suggests, is an underground stone-lined tunnel, typically constructed during the early medieval period in Ireland as a place of refuge, cool storage, or concealment. This one runs roughly east to west, its visible length just 3.6 metres, its width 1.4 metres, and its depth no more than 0.8 metres where it can be seen at all. Flat stone lintels cap what remains accessible. Hazel trees have long since colonised the western end, and at the eastern end there appears to be a circular chamber that has collapsed inward, leaving the whole structure sealed as much by time as by vegetation.

The cashel itself, a cashel being a stone-walled ringfort of the early medieval period, forms the broader context here. The souterrain was built into the cashel's north wall, close to the north-west corner, which suggests a deliberate integration of the two structures rather than a later addition. What complicates the picture further is what came after. A drystone wall roughly two metres in diameter was built around the collapsed chamber at the eastern end, and adjoining this is a house of indeterminate date, its walls actually overlying the cashel wall beneath. The sequence of occupation visible here, cashel, souterrain, later enclosure, later still a dwelling of uncertain age, speaks to a site used and reused across centuries, each layer pressing down on the one before it.