That small, partially blocked opening, known as a creep, was an architectural feature of early medieval souterrains, the underground stone-lined passages and chambers built by Irish communities as places of refuge or storage. The one at Crumlin is more than usually interesting because of the detail that survives in its surviving sections, despite the damage done by centuries of stone-robbing.

The structure sits in the north-western sector of a badly deteriorated cashel, a type of dry-stone walled enclosure used as a farmstead in early medieval Ireland, which itself lies immediately north-east of a feature known locally as Crumlin Fort. By 1915, when the Cassini edition of the Ordnance Survey six-inch map was produced, the souterrain had been reduced in local memory to a simple label: Cave. The complex follows a roughly L-shaped plan. An open, roofless hollow running south-west to north-east, around 4.5 metres long, represents what was once a roofed passage or chamber whose covering stones were taken away at some point. At its north-eastern end, a surviving roofed chamber turns ninety degrees to run south-east. Its dry-stone side walls corbel gently inward from floor level to carry large lintel stones across the top, though those lintels are now subsiding at the chamber's centre. The floor rises towards the south-eastern end, forming a raised rectangular platform about a metre high, and set into that platform is the creep, giving crawl-through access to a further chamber beyond. That second roofed chamber, around six metres long and covered by five large stone lintels, is entered from an opening at its south-eastern end and still contains a considerable amount of loose stone on the floor. A small square opening in the dry-stone walling of the open hollow, only twenty centimetres in each direction but traceable for about 1.5 metres into the structure, may have functioned as an air-vent.

The creep is the detail worth pausing over. Its dimensions, roughly ninety centimetres long, seventy wide, and only thirty-five deep, mean that anyone passing through it was momentarily defenceless, unable to stand or turn quickly. This was the point. The design forced intruders into a vulnerable position while those inside retained the advantage. That a structure this specific and this carefully considered still exists, even partially, within a cashel that is itself described as very poorly preserved, makes the Crumlin souterrain quietly remarkable.