What it revealed was a souterrain, an early medieval underground construction of the kind found widely across Ireland, typically consisting of stone-lined or earth-cut passages and chambers dug beneath a farmstead. They were used for storage, refuge, or both, kept naturally cool and concealed from view. The Knocknagoul example is earth-cut rather than stone-lined, which places it among the less elaborate but no less intriguing examples of the type.

Investigated by R. M. Cleary in 1989, the souterrain consists of two roughly rectangular chambers connected by a low, narrow passage called a creepway, designed to be crawled through and easily defended. The first chamber measures approximately 3.4 metres long and 1.6 metres wide, running on a north-south axis, though its original height cannot now be determined due to collapse. A second creepway leads from the eastern side of this chamber into the second, which is slightly narrower at 1.5 metres wide and just one metre high, with a crudely paved floor and its south-western end choked with loose stones and soil from what appears to be a collapsed construction shaft. Most intriguingly, a blocked creepway in the middle of the western wall of the first chamber has never been fully investigated; Cleary noted that it may lead to additional chambers or to the structure's original entrance, meaning part of this underground system remains unexplored. Roughly 35 metres to the west lies a fulacht fiadh, a type of ancient cooking site associated with heated stones and water troughs, suggesting this was once a more densely used part of the landscape than the quiet farmland it appears today.