It is not an entrance to the enclosure itself. Instead, it opens onto an underground passage that travels inward and then turns, suggesting a route that once connected the outside world directly to the interior of the site by way of the ground beneath the walls. That kind of subterranean corridor is known as a souterrain, a stone-lined underground tunnel typically associated with early medieval ringforts and cashels in Ireland, most likely used for storage, refuge, or both.

A cashel, or caher, is a stone-walled enclosure of the early medieval period, roughly the fifth to twelfth centuries, and Cill Rialaigh sits within a landscape on the Iveragh Peninsula unusually dense with such remains. The souterrain here was noted by a researcher named Lynch in 1902, who recorded that the passage, after bending to the north-west, rose back up to the level of the cashel's interior. That internal opening, however, cannot now be located. The passage itself can be traced for at least two metres from the external entrance before the bend, but beyond that point access has been lost, whether through collapse, silting, or deliberate blocking is not clear.