The surviving structure is modest: a two-storey rectangular building of random-rubble limestone, measuring roughly five and a half metres by four and a half metres internally, with vegetation pressing in from the north and a low gable still visible on the south wall. Alongside the southern elevation sits a shallow wheel-pit, about one and a half metres wide and less than a metre deep, which almost certainly housed an undershot waterwheel, the kind driven by water flowing beneath the wheel rather than over it. A shallow, wet, earth-cut channel curves around the west, north, and east sides of the building, most likely the remnant of a millrace, the artificial channel used to direct river water to the wheel.

What makes the site more than a modest ruin is the layered history behind it. The Down Survey of 1655 to 1656, a remarkably detailed mapping project carried out under William Petty following the Cromwellian settlement of Ireland, recorded a mill at this location and a tuck mill a short distance to the west. A tuck mill, also known as a fulling mill, was used to clean and thicken woollen cloth by beating it with heavy wooden hammers, a process entirely distinct from grain milling. By 1842, when the Ordnance Survey produced its six-inch maps, the complex had grown: the surviving structure is shown alongside two considerably larger buildings to the north, and the whole is labelled Castlemagner Mill. Those larger buildings had vanished by the time the 1905 Ordnance Survey revision was made. Writing in 1934, a researcher named Bowman noted that the ruins of both grinding and tuck mills were still visible, but recorded that neither had been in use since the Famine, placing their abandonment somewhere in the late 1840s. The mill had apparently stood idle for the better part of a century by then, its working life ended by the same catastrophe that emptied so many corners of rural Ireland.