A souterrain is an underground passage or chamber, typically stone-lined, built during the early medieval period and associated with ringforts, the circular enclosed settlements that dot the Irish countryside in their thousands. They were used variously for storage, refuge, or concealment. This one, lying on the south side of the ringfort at Knockagolig, was found, investigated to some degree, and then closed. It has been invisible ever since.

The only witness in the record is a 1934 reference by Bowman, who noted that the souterrain had been discovered "some years" before that date and had by then already been sealed. That phrase, vague as it is, places the discovery somewhere in the late nineteenth or early twentieth century, a period when agricultural work and land improvement frequently broke open underground structures that had lain undisturbed for a millennium or more. The ringfort it belongs to is still recorded, but the souterrain itself exists now only as a written note, a reference embedded in a sentence written ninety years ago about something that had already been closed.