On its own it might pass for nothing more than field debris, but its position, just inside the bank of the rath, and its dimensions, roughly 0.8 metres long and 0.4 metres deep, suggest it may once have served as a lintel, the capstone spanning the entrance to a souterrain below.

A souterrain is an underground passage or chamber, typically built from dry-stone walling and roofed with large flat slabs. They appear throughout early medieval Ireland, often associated with ringforts, and were used variously for cold storage, refuge, or concealment. The Mount Temple rath itself is a ringfort, a type of enclosed settlement common between roughly the fifth and twelfth centuries, consisting of a circular area bounded by an earthen bank and ditch. What lies beneath the fallen stones here is uncertain. The entrance, if that is what it is, appears to have been blocked at some point, and whatever passage or chamber might exist beyond it remains unexcavated and largely unknown.