A stone-lined passage, less than a metre wide and only seventy centimetres high, extends northward into the earth, its ceiling formed from flat lintels laid across the walls in a technique that has kept structures like this intact for over a thousand years. You can follow it with your eyes for about three metres before the darkness takes over, but the passage almost certainly goes further.

A souterrain is an underground stone-built passage or chamber, typically constructed during the early medieval period in Ireland, probably serving as a refuge, a cool store for perishables, or both. This one sits within its parent ringfort, the kind of circular earthwork enclosure that once served as a farmstead for an early Irish family, and the relationship between the two structures is characteristic: souterrains are frequently found tucked inside ringforts, accessed from within the protected interior. At Glen, the passage leads toward at least two stone-lined chambers, roofed in the same lintelled style as the entrance, though these were explored in the past rather than formally excavated. Their exact condition now is less certain. Three distinct depressions in the bank to the northwest of the opening are a quieter piece of evidence, most likely marking places where the roof of the souterrain has given way beneath the surface, leaving the ground above to sink.