A souterrain, the term for an underground stone-built passage typically associated with early medieval settlement, once ran beneath this site at Ballyfaris, and what visitors or surveyors encounter today are the depressions and hollows left behind as sections of the structure have given way over centuries.

The souterrain sits within a rath, a circular earthen enclosure of the kind built across Ireland during the early medieval period as a defended farmstead. By the time the Ordnance Survey mapped the area in 1913, the passage had already receded so far from common knowledge that it was marked simply as "Cave" at the south-eastern edge of the rath, a label that captures the local impression of it rather than its archaeological character. What the ground preserves today is a series of irregular sunken features: a linear hollow running roughly north-west to south-east in the south-eastern quadrant, measuring around 8.4 metres in length and sunk to a depth of about 0.6 metres, thought to represent a collapsed section of the souterrain itself. In the south-western quadrant, two further features complicate the picture. One is a longer linear depression, some 10 metres in extent and defined along its north-eastern edge by a low earthen bank, which may point to another section of the underground system or a related feature. Immediately to its north sits a circular hollow roughly 3 metres across and 0.8 metres deep, the purpose of which remains less certain.

Taken together, these depressions map out a structure whose full extent is still partly hidden, the ground offering clues rather than conclusions.