The structure at Killossery manages this by stepping from two storeys down to one as it ascends the slope, its roughly north-south orientation following the natural contour of the land rather than fighting it. The thatched roof, now sheathed in galvanised iron to preserve what lies beneath, gives the building an odd layered quality, old materials held in place by newer ones, a kind of accidental conservation.

The mill's documented history reaches back to the mid-seventeenth century. The Civil Survey of 1654 to 1656 records a mill at Killossery in the ownership of Philip Hore of Kilsallaghan, and the same mill appears on the Down Survey map of 1655 to 1656, the great Cromwellian mapping project that recorded land ownership across Ireland at a moment of profound political upheaval. The complex that survives today comprises the mill building itself to the west of a yard, a dwelling house set at right angles to it, and some later outbuildings to the north. The mill building has stone foundations with mud walls and hipped gables, meaning the roof slopes down on all four sides rather than ending in a flat vertical face, and it measures roughly 16.4 metres in length by 6.6 metres in width. A millrace, the channel cut to direct water onto the wheel, runs along the northern gable, and most of the original mill machinery survives in the northern end of the building. The southern end contains a kiln, likely used for drying grain before milling.

The complex sits to the north of Killossery Church, tucked into the hillslope in a way that makes it easy to pass without registering what you are looking at. The relationship between the mill building, the dwelling, and the outbuildings is worth taking time to read as a whole, since the layout reflects the practical logic of a working agricultural complex rather than any designed arrangement. The surviving mill works in the northern section are an unusual find; internal machinery of this kind rarely lasts.