The landowner has long maintained that a stone-capped cavern or creepway runs under the tangled interior of the enclosure. Surveyors, on the other hand, found no trace of it when the site was cleared in the 1990s. What remains is an unresolved disagreement between oral knowledge and the spade, which is itself a fairly common situation when it comes to souterrains, the narrow underground passages, often stone-lined and stone-roofed, that were built in association with early medieval settlements across Ireland. Their purposes are debated, with storage, refuge, and drainage all proposed at various times, but their habit of collapsing, silting up, or simply being forgotten about is well established.

The rath at Knockasarnet is a ringfort, a type of enclosed farmstead typically dating from the early medieval period, roughly the sixth to the tenth centuries, though many were used for longer. This particular example contains a linear feature running on a northwest to southeast axis, roughly five metres long and three metres wide, which abuts the internal face of the inner bank at its eastern end. That alignment and position are consistent with a souterrain, and may represent part of one. When the interior of the rath was cleared during construction of a house to the east of the site in the 1990s, however, no physical evidence of the underground passage came to light. Whether the clearance missed it, whether it had already collapsed beyond recognition, or whether the landowner's account preserves a memory of something now entirely gone, the site holds the question open.