The excavators who uncovered it in 1929 noted the pragmatic logic of that choice without apparent irony. This is a souterrain, one of two found within a ringfort at Togherstown; a souterrain being an artificial underground passage or chamber, typically built in early medieval Ireland for storage or refuge. The one excavated in 1929 and 1930 was the smaller of the two and was called the Lesser Souterrain by the team who dug it.

R. A. S. Macalister and R. Lloyd Praeger published their findings in 1929 to 1931, and their description reads like directions through a puzzle. The entrance, formed by that repurposed boulder alongside a large lintel stone, opens into an oval chamber roughly 5.2 metres long and 1.57 metres wide, its walls still standing to a height of nearly 1.4 metres. This chamber was cut through the inner fosse, the defensive ditch of the surrounding ringfort, which had to be partially backfilled to accommodate it. Beyond the chamber, a series of passages contracts and bends, with jamb-stones creating narrow constrictions that a person would have to squeeze through, a deliberate design feature common to souterrains, probably intended to slow or discourage unwanted visitors. A small cupboard-like recess was found roughly halfway along one of the longer passages. At the far end, two separate doorways, both low and awkward, admit to a pair of roofed chambers, each covered with large flagstones. The better-built of the two, chamber F, measures about 4.4 metres long, and has a small wall recess at its inner end. Macalister and Praeger found it easier to break through the clay behind that recess than to continue using the proper entrance. Traces of large fires were recorded in and around the original entrance structure, though what those fires signified was not determined.