What makes it worth pausing over is that small circular depression in the south-west quadrant, roughly three metres across and forty centimetres deep, with no agreed explanation for what it once held or served. Everything else about the site is legible; that hollow is not.

The enclosure is a rath, the most common monument type in the Irish countryside, built during the early medieval period, broadly the fifth to twelfth centuries, as a defended farmstead for a single family or small community. A rath typically consists of a raised circular area surrounded by an earthen bank and an outer ditch, and this example follows that pattern precisely. The enclosed area measures forty-seven metres in diameter, ringed by a bank of earth and stone about three metres wide, with an external fosse, or ditch, of similar width and roughly eighty centimetres deep. The original entrance survives on the southern side as a deliberate three-metre gap in the bank, accompanied by a causeway crossing the fosse, which is exactly how such sites were designed to be entered. The location on a ridge slope overlooking the surrounding land would have given the inhabitants a useful degree of visibility, and the Ox Mountains rising behind would have provided some shelter from the prevailing weather to the north-west. What remains unexplained is that shallow, circular hollow in the south-west interior. Features like it on other ringforts have been interpreted variously as the footprint of a souterrain, an underground stone-lined passage sometimes used for storage or refuge, or as the site of a sunken-floored structure, but at Ballinphull its function is recorded simply as uncertain.