A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts and cashels, and often interpreted as a place of refuge or storage. The one at Caheradine was, by one account, unusually elaborate, said to consist of three rooms, which would make it a more complex example than the single-passage tunnels more commonly recorded.

The earliest written reference comes from Redington, writing in 1916, who noted that the souterrain lay within the cashel and had already been filled in by that point. When McCaffrey investigated the site in 1952, he could find no trace of it at all. A later account from the landowner added a practical explanation: he had himself blocked up a souterrain in the northern half of the monument. Whether this is the same feature Redington described, or a separate one, the notes do not make clear. What the sequence of references does suggest is a site that has been gradually closed off over the course of the twentieth century, its underground rooms becoming first inaccessible, then invisible, then effectively absent from the landscape entirely.