The souterrain at Kiltiernan is not visible in any conventional sense, but its dimensions alone suggest something worth pausing over: more than ten metres long, roughly a metre wide, and just 0.7 metres high, it is a passage built not for comfort but for purpose, close enough to the ground that anyone moving through it would have had to crawl.

A souterrain is an underground structure, typically dry-built from unmortared stone, associated in Ireland with the early medieval period. They appear frequently within or adjacent to raths, the circular earthwork enclosures, sometimes called ringforts, that are among the most common archaeological features in the Irish landscape. Their function is debated, but storage, refuge, and ventilation have all been proposed. At Kiltiernan, the souterrain sits in the north-west quadrant of just such a rath, and its relationship to the enclosure above is typical of the type. Collapsed slabs at the eastern end suggest the passage once extended further in that direction, and there may have been a second chamber branching off to the north near that same end. McCaffrey noted the site in 1952, placing it among a catalogue of similar structures recorded across the region.

The site is recorded as inaccessible, and the subterranean nature of souterrains means that even where they survive intact, the ground above can give little indication of what lies beneath.