A souterrain is an underground passage or chamber, typically stone-lined, built during the early medieval period and associated with ringforts across Ireland. They served various purposes, including storage and refuge. At Clashanure, there is no longer any surface trace of it whatsoever.

The sole record comes from P. J. Hartnett, who visited in 1939 and noted, with careful vagueness, that to the south-west of the centre of the ringfort was "what I was told was a souterrain which has been closed up". The phrasing is telling. Hartnett did not see it himself; he was reporting local knowledge, secondhand, of something already sealed. Whether it was deliberately blocked, collapsed, or simply absorbed back into the ground, the structure had already withdrawn from view before he arrived. The ringfort it belongs to is recorded separately, but the souterrain exists in the archaeological record largely as a rumour someone passed on to a researcher more than eighty years ago.