The main structure, a multi-bay two-storey limestone block built around 1800, now serves as agricultural and domestic outbuildings, its natural slate roof largely intact while the former corn kiln alongside has acquired a corrugated-iron replacement. A small mill race still runs to the south side of the building, though the water wheel it once served has long since gone. What makes the place worth pausing over is not simply the architecture but the suggestion of continuity: the same spot was almost certainly processing grain centuries before the present walls went up.

The Down Survey, a mid-seventeenth-century mapping project that recorded land ownership across Ireland following the Cromwellian wars, noted a corn mill at Killagh alongside a tuck mill, a ruined church, and a castle described as being in repair. The castle at that time stood on lands belonging to George Nugent, recorded in 1641 as an Irish papist, a designation used in the Survey to identify Catholic landowners whose holdings were subject to confiscation. The 1838 edition of the Ordnance Survey six-inch map shows a corn mill on what is very likely the same site. By around 1850 the mill was in the hands of a John Heggarty, and the Valuation Office Mill Book records it as serving a dual purpose: corn milling and tuck milling, the latter referring to the processing of woollen cloth. Its water wheel measured ten feet in diameter and its water power was valued at £6 and 15 shillings. The kiln, set at a right angle to the northwest of the main building, would have been used for drying grain before grinding, a standard arrangement in Irish corn mills of the period. The structure itself is coursed rubble limestone with dressed limestone quoins at the corners, segmental-headed carriage arches, and brick voussoirs, the wedge-shaped stones that lock an arch in place, lending the openings a slightly more refined finish than the rough-hewn walls around them.