These mysterious underground passages, found throughout Ireland, were constructed during the early medieval period and likely served as storage spaces, refuges, or both. The Carrowcashel souterrain, though partially filled with clay and stones over the centuries, still reveals the ingenuity of its builders.

Entering through the northern opening, which measures 1.6 metres wide, visitors would need to crouch considerably as the passage now stands only 1.2 metres high. The construction technique shows a fascinating blend of methods; whilst the upper walls consist of carefully placed dry stone work, the lower sections appear to have been carved directly from the bedrock, suggesting the builders took advantage of the natural geology. The roof overhead consists of substantial stone slabs, a typical feature of souterrain construction that has allowed these structures to survive for over a millennium.

The passage narrows dramatically as one moves deeper underground, constricting from its original width to just half a metre near what appears to be the current terminus. Here, a bank of clay and stones blocks further progress, leaving archaeologists to wonder whether the souterrain originally extended even further into the darkness. This example was documented during the comprehensive Archaeological Survey of County Donegal in 1983, joining hundreds of similar sites that offer tantalising glimpses into how medieval Irish communities lived, stored their goods, and protected themselves during uncertain times.