A circular hollow, roughly two and a half metres across and about forty centimetres deep, sits in the north-eastern quadrant of an early medieval rath, and its shape strongly suggests that something below it has collapsed inward. The most likely culprit is a souterrain, one of the stone-lined underground passages or chambers that were built beneath and around Irish raths during the early medieval period, probably used for cold storage, refuge, or both.

A rath, sometimes called a ringfort, was the most common form of rural settlement in early medieval Ireland, typically consisting of a circular area enclosed by one or more earthen banks and ditches. Souterrains were frequently constructed within or adjacent to these enclosures, their corbelled stone roofs or lintelled passages vulnerable to collapse once the structures fell out of use and maintenance ceased. At Carrowcashel, no excavation appears to have taken place, and the feature remains unconfirmed. The hollow is interpreted cautiously as a possible indication of a chamber beneath, its circular form and modest depth consistent with what a collapsed roof-void might leave behind at the surface. Without breaking the ground open, the full extent and character of any underground structure here remains unknown.