A souterrain is an underground stone-lined passage or chamber, typically constructed during the early medieval period and associated with the ringforts that once dotted the Irish countryside. They served variously as places of refuge, storage, or concealment. The one at Boolymore has been filled in, and the ground above it shows no trace of what lies beneath.

The only record of this structure comes from a 1934 reference by Bowman, who noted that a souterrain had been found there "many years hence" but was already filled in by that point. Which means the infilling predates 1934 by an unspecified stretch of time, and the original discovery predates that still further. The souterrain sits within a ringfort, the kind of enclosed farmstead that was the dominant form of rural settlement in early medieval Ireland, typically consisting of a circular area bounded by one or more earthen banks and ditches. The combination of ringfort and souterrain is common enough across Ireland, but at Boolymore the underground element has effectively ceased to exist as an accessible feature, leaving only the documentary trail and the faint possibility that something still holds its shape below the soil.