A circular enclosure, marked simply as "Fort" on the 1837 edition of the six-inch Ordnance Survey map, once occupied a low rise on an otherwise flat stretch of land, the kind of subtle elevation that often drew prehistoric or early medieval settlement precisely because it offered a degree of visibility and drainage. That modest eminence, and whatever earthworks sat upon it, are now submerged beneath a reservoir.

The fort at Lisfarrell belonged to a category of monument common across Ireland: a roughly circular enclosure, likely defined by an earthen bank and ditch, of the type often called a ringfort or rath. These enclosures date broadly to the early medieval period, though some have earlier origins, and they functioned variously as enclosed farmsteads, status markers, or defended homesteads. By the time the Ordnance Survey teams passed through Longford in the 1830s, many such sites had already been reduced by centuries of agriculture, but enough survived to be recorded on the new maps. The Lisfarrell example was noted on a rise, a detail that suggests it would once have been a visible feature in the local landscape. At some point after that survey, the construction of a reservoir erased it entirely, leaving the 1837 map as its primary documentary trace.