The depression, roughly a metre wide and extending some four metres westward from a collapsed entrance, traces the probable line of a souterrain, an earth-cut or stone-lined underground tunnel of the sort commonly built beneath or beside the raised earthen enclosures known as raths. A rath, sometimes called a ringfort, was a farmstead defined by one or more circular banks and ditches, and souterrains associated with them were typically used for storage or as places of refuge. Here, the entrance, positioned at the internal base of the rath's bank on its eastern side, still shows a rounded roof where the ground has not yet fully given way, but the passage beyond it has long since become impassable.

What makes the Carhoomeengar example quietly interesting is the paper trail it carries. When fieldworkers recorded the site in the 1940s, the souterrain was apparently in better condition, noted at the time as featuring a stone leading into a passage. That detail, modest as it sounds, suggests the entrance was still at least partially intact within living memory. By the time more recent survey work examined the site, the interior had become inaccessible, and what survives above ground amounts to little more than a slight hollowing of the earth, following the buried corridor beneath.