A souterrain, to give the structure its proper name, is an artificially constructed underground passage or chamber, typically built during the early medieval period in Ireland and associated with the ringforts, or circular enclosed farmsteads, that were once the dominant form of rural settlement across the country. Their exact purpose is still debated, though food storage and refuge in times of threat are the explanations most commonly put forward. The one at Killagh More is modest in scale but intact enough to be legible: a drystone-built chamber, meaning its walls are constructed from stones laid without mortar, running roughly north-northwest to south-southeast and measuring 3.3 metres in length, with its present point of access at the north-northwestern end.

What makes the Killagh More example quietly interesting is the possible presence of a second feature nearby. Some 13.5 metres to the northwest of the souterrain lies a stone-lined depression, subrectangular in shape and considerably larger at 8 metres in length, oriented west-northwest to east-southeast. Whether this depression is directly related to the souterrain, or simply another element of the same broader settlement complex within the ringfort, is not certain; the two are described as possibly associated. That careful hedging is characteristic of how archaeologists treat sites where the ground has not been excavated and the evidence remains ambiguous. The ringfort itself, recorded separately, would originally have consisted of an earthen or stone enclosure encircling a domestic space, and the souterrain would have sat within or beneath that protected interior.