A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period as a place of refuge, storage, or concealment within or beside a ringfort. At Islandav, there is no visible surface trace of it whatsoever.

What is known comes from a single reference: Bowman, writing in 1934, noted the site of a souterrain on the north-east side of the ringfort with which it is associated. That ringfort still exists as a recorded monument, and the pairing of the two features would have been entirely typical of early medieval Irish farmsteads, where underground chambers were routinely dug to provide cool storage for dairy produce or a retreat in times of danger. Whether Bowman observed a physical remnant or was working from local tradition or earlier records is not stated, and nearly a century on, nothing remains visible to confirm the identification.