This is a souterrain, an underground stone-lined tunnel typically associated with Irish ringforts and cashels, used for storage, refuge, or both. What makes this particular example quietly remarkable is not its dimensions, modest as they are at six metres long and less than a metre across, but what was found inside it when excavators finally got a proper look.

When Hugh Hencken excavated the site in 1938, he uncovered a layer of ash and animal bone within the passage, and at one end, a human skull carefully surrounded by flat stones. He interpreted this as a foundation deposit, the deliberate placement of human remains during or shortly after the souterrain's construction, a practice meant to consecrate or protect the structure. It is an arresting idea, but one that has not held up especially well. Writing in 1999, Claire Cotter pointed out that no known parallels exist for such a theory in the Irish archaeological record, and that the burial is more plausibly explained as an interment made after the souterrain had fallen out of regular use, the underground space repurposed, as often happened, for the dead. A second find complicates the picture further: a silver annular brooch, a type of ring-shaped fastening common in early medieval Ireland, was recovered from the roofed southern end of the passage. Raghnall Ó Floinn has suggested the brooch was most likely deposited in the ninth century, placing at least some activity in the souterrain within the historically turbulent decades of Viking-age Ireland.

Cahercommaun itself is a national monument in State care, set into a dramatic cliff edge above a valley in the Burren. The souterrain sits in the northern portion of the fort, adjacent to a second souterrain on the same site, and visitors exploring the cashel's interior can observe the limestone slab roofing that is characteristic of the construction. The steps down into the passage give some sense of how deliberately engineered these spaces were, even if the full interior is no longer accessible in the way Hencken's team experienced it.