A souterrain is an underground passage or chamber, typically dry-stone lined, built during the early medieval period and associated with ringforts and cashels across Ireland. They were used variously for storage, refuge, or ventilation of dairy goods. The one at Cahercrin has not been seen in living memory, smothered as it is beneath dense overgrowth, trees, and scrub.

Two early twentieth-century antiquarians put the feature on record. Redington, writing in 1916, and T. J. Westropp, writing in 1919, both noted the souterrain as lying within the interior of the cashel, the term for a stone-walled ringfort of the early medieval period. Neither account elaborates beyond placing it there, and when fieldworkers later visited the site they could not locate it at all. The vegetation had simply taken over. That gap between historical record and physical reality is not unusual in Irish field archaeology, but it is rarely quite so total: a structure that has been written about twice and found not once.