A souterrain is an underground stone-lined passage or chamber, typically associated with early Christian period settlement in Ireland, and thought to have served as a place of refuge, cool storage, or both. What came to light at Tisaxon was the narrowest and lowest section of such a structure, the part known as a creep, a deliberately constricted portion of the passage that would have forced anyone moving through it to do so on hands and knees.

The exposed section comprised two low runs of vertical walling, with an external width of 1.5 metres and standing to about a metre in height. The tops of the walls lay only 0.3 metres beneath the ground surface, and the eastern wall still appeared to carry a partially displaced lintel stone across its top. Above ground, a shallow rectangular depression running northward from the creep may represent the subsided track of the fuller souterrain beneath, hinting that more of the structure remains unexcavated. Roughly 24 metres to the south-west, also exposed in the quarry face, human remains were found, a burial whose relationship to the souterrain is suggestive but, at the time of recording, unresolved. The two discoveries together point to a site of some complexity, the kind of layered early medieval landscape that tends only to become visible when the ground is disturbed in ways nobody planned for.