The entrance measures 0.7 metres wide and 0.5 metres tall, dropping almost half a metre into a corridor so confined, at an average 0.7 metres by 0.6 metres, that movement through it requires something closer to crawling than walking. At two points along its length, projecting roof lintels narrow the passage still further, creating what archaeologists call creep features, deliberate constrictions that force anyone moving through to slow and lower themselves even more. Whether these were defensive measures, symbolic thresholds, or simply the result of structural necessity is not entirely settled.

This is a souterrain, a type of underground stone-built structure found widely across early medieval Ireland, typically associated with nearby settlement and used for storage, refuge, or both. The one at Knockglass More is dry stone construction throughout, its walls built without mortar and its roof formed from flat stone lintels laid across the passage. What sets it apart is a detail on the floor: stone cobbles, clearly gathered from the shore immediately adjacent to the site. Someone, at some point, carried those stones underground and laid them underfoot. A natural gully formed by water erosion has since cut into the northern end of the souterrain, exposing the curved terminal end of the passage and leaving the entrance open to the surface, which is how it remains visible today.