What they indicate is a souterrain, an underground passage or chamber built, typically in the early medieval period, beneath or adjacent to a defended settlement. This one is now inaccessible, sealed by centuries of collapse and infill, and sits quietly within the north-west quadrant of a rath, the ringfort that once surrounded it.

A rath, to use the term as it was understood across early medieval Ireland, was a circular enclosure defined by an earthen bank and ditch, serving as a farmstead and a statement of local status. The souterrain beneath this one at Aggard Beg would have functioned as a place of storage, refuge, or concealment, connected to the domestic life of whoever occupied the enclosure above. McCaffrey noted the site in 1952, recording its location and the fragmentary evidence still visible at ground level. That a roof lintel remains exposed at all suggests the passage was once more substantial, its stonework cut and laid with some care before the structure eventually gave way.