At Rosnacartan Beg in County Kerry, what was once described as a cave and an underground passage now leaves no trace above ground, which makes it an oddly compelling entry in the record of Irish antiquities. The feature in question is classified as a souterrain, a type of man-made underground structure typically associated with early medieval settlement, consisting of one or more passages and chambers cut into the earth or built from stone and then covered over. They were used variously for storage, refuge, or both, and are found throughout Ireland wherever raths, the enclosed farmsteads of early medieval farmers, once stood.

The souterrain at Rosnacartan Beg was noted in the 1940s as lying within a large trivallate circular rath, meaning a rath defended by three concentric banks and ditches rather than the more common single or double circuit. That triple enclosure suggests a site of some local significance. At the time, the land belonged to a Mr. Clifford, and whoever recorded the feature described it in straightforward terms: a cave and an underground passage. Whether the structure was explored, measured, or simply noted in passing is not clear, but no visible remains of it survive today.