A souterrain is an underground chamber or tunnel, typically constructed during the early medieval period in Ireland, built from drystone walling and roofed with flat slabs. They served various purposes, most likely as places of refuge, cool storage, or escape routes. The one at Ballyturin sits inside a cashel, a type of circular stone enclosure that functioned as a defended farmstead, and it has left nothing above ground to betray its presence.

In the early 1960s, a depression that opened up in the centre of the cashel drew enough attention to be investigated, and what was found was described at the time as a tunnel or cave that was stone and flag built. That brief description is almost all that survives by way of documentation. The cashel itself remains a separate recorded site, but the souterrain has since lost any visible surface trace. Whether the depression was partially explored and then left, or whether settling ground simply reclaimed whatever opening had appeared, is not recorded. It exists now largely as a notation of something once glimpsed and then swallowed back into the earth.