This is a souterrain, a type of structure built throughout early medieval Ireland, typically during the first millennium AD, as a place of refuge, storage, or both. The Lissyegan example is poorly preserved, its south-eastern access point robbed out over centuries, the stones taken piecemeal for walls or buildings elsewhere, as happened to so many such structures across the country.

The passage aligns roughly north-north-west to south-south-east and sits in the west-north-western sector of the associated ringfort, a circular enclosed settlement defined by an earthen bank and ditch. What makes the arrangement here particularly interesting is that the northern end of the souterrain appears to abut or run beneath the inner face of the ringfort's own bank, integrating the underground passage directly into the structure of the enclosure itself. There is also evidence of a possible air vent at that northern end, a feature sometimes found in souterrains to prevent the build-up of stale air in the chamber below, suggesting this passage may once have been a functioning and reasonably sophisticated underground space rather than a simple pit or crawlway.