A souterrain, as these features are known, is a type of man-made underground passage or chamber associated with early medieval ringforts across Ireland, most likely used for storage, refuge, or both. What makes the one at Lisladeen particularly intriguing is not what is known about it, but what remains deliberately, or at least obstinately, unknown.

The souterrain came to attention in 1978 when part of it collapsed, prompting an investigation by J.P. McCarthy. What he found was a stone-built chamber oriented north-west to south-east, measuring at least 1.6 metres in length, just under a metre wide, and roughly 0.9 metres high, with walls built in a steeply corbelled style, meaning the stones were laid so that each course projects slightly inward, creating a rough vault without mortar. The chamber was partially filled with soil and stones. At its north-western end, a narrow creepway, the kind of low connecting passage sometimes found linking chambers in more complex souterrains, ran off at a right angle. This passage measured less than half a metre in width and just 33 centimetres in height, and was blocked with stones, making it impossible to explore further. McCarthy noted that the infilling appeared to have been deliberate rather than the result of simple decay or collapse, and suggested the structure is either a single chamber with a blocked side passage, or an L-shaped souterrain with additional chambers lying beyond the blockage, still unexamined. The site has since been closed up, leaving that question unanswered.