The uncertainty is not carelessness; it is the condition of the site itself, which has resisted close inspection for well over a century.

A souterrain is an underground passage or chamber, typically built during the early medieval period, used variously for storage, refuge, or ventilation of adjacent structures. The one, or ones, at Caherbulligin sit within a cashel, a type of stone-walled circular enclosure that served as a farmstead or settlement in early Christian Ireland. Writing in 1916, Redington noted two filled-in souterrains within the cashel. By 1952, McCaffrey recorded the presence of a souterrain in the interior but could not say more, because the site was so densely overgrown with thorn and scrub as to be completely inaccessible. Whether anything changed between those two accounts, or whether the discrepancy reflects the difficulty of surveying a site already half-consumed by vegetation, is impossible to say with confidence.

What the record leaves behind, then, is a place defined largely by what cannot be seen. The thorn and scrub that defeated McCaffrey's inspection in the early 1950s have had decades more to consolidate their hold, and there is no indication that the interior has since been cleared or properly examined.