A souterrain is an underground stone-lined passage, typically associated with early medieval ringforts, or raths, and used variously for storage, refuge, or concealment. The one at Cummers belongs to that quietly melancholy category of archaeological sites that have been, in effect, deliberately erased from the surface of the landscape, not by time or neglect, but by a very practical human decision.

According to local information, the entrance to the underground passage was blocked up some years ago to stop animals from getting in and injuring themselves. It is the kind of pragmatic intervention that would have seemed entirely reasonable at the time, and probably was, but it has left the site in an ambiguous state. The souterrain sits within a rath, a circular earthwork enclosure of the sort built throughout Ireland during the early medieval period, and the two features together would once have formed part of a functioning farmstead. Whether the passage survives intact beneath the sealed entrance, or has partially collapsed, is simply not known.