A breach was made at the south-western end at some point in the modern era, but the interior remains inaccessible, leaving the structure to exist as a kind of geological rumour beneath the ground.

What lies beneath is almost certainly a souterrain, an underground construction of dry-stacked stone, typically built during the early medieval period in Ireland as a place of refuge, storage, or concealment. A souterrain usually consists of one or more corbelled chambers connected by low, narrow passages, and this example at Cregg appears to have been no modest affair. When the antiquarian G. H. Kinahan described the site in 1883 to 1884, he recorded a passage leading to somewhere between three and four chambers, which would make it a relatively substantial example of its type. The drystone-built passage that can still be traced today runs on a north-east to south-west axis, with a length of more than 6.3 metres measurable, though the full extent of the underground network is difficult to determine without access to the interior.