What he found was modest in scale but precise in its construction: a curved passage running roughly northeast to southwest, measuring sixteen feet in length and less than three feet high at its narrowest recorded point, with side-walls of drystone masonry corbelled slightly inward so that overlapping roof slabs could be laid across the top. At both ends, the passage met further blocked sections, suggesting the accessible portion was only a fragment of a larger, largely sealed system.

The souterrain sits within a rath, the circular earthen enclosure that served as a farmstead enclosure during the early medieval period in Ireland, roughly the fifth to twelfth centuries. Such enclosures frequently concealed souterrains beneath their interiors, and Cooke gained entry from a point close to the centre of the rath, which is itself unusual; access points were more commonly positioned near the perimeter. The two blocked passages, one extending towards the southeast from the northeast end, and one closing off the northwest end of the curve, remain unexplored, meaning the full extent of the underground network at Clydagh is still unknown. The corbelled construction, in which stones are laid so that each course projects slightly beyond the one below it to narrow the gap until roof slabs can bridge it, is a technique found in souterrains across Munster and points to considerable skill in dry-stone building without the use of mortar.