That cautious parenthetical, "site of", is doing considerable work. It signals that even by the early twentieth century, whatever once existed here had already passed out of visibility, leaving only a cartographic memory.

The feature in question is a souterrain, an underground stone-lined passage or chamber typically associated with early medieval ringforts, used variously for storage, refuge, or ventilation. This particular example sits in the south-eastern quadrant of a rath, the Irish term for a roughly circular earthen enclosure that served as a farmstead during the early medieval period. The pairing of souterrain and rath is common enough across Ireland, but the combination at Lisbanagher is now essentially invisible at ground level. No structural remains have been recorded as surviving, and the OS map itself framed the feature as a lost site rather than an active one, suggesting the collapse or infilling had occurred well before the surveyors arrived to document it.