Its presence here, built into the south-east wall as though salvaged and reused rather than discarded, is a small but telling detail: somebody once thought it worth keeping, even as the building around it fell away.

Three walls of the rectangular mill survive, enclosing a space roughly 12.6 metres by 15.3 metres, with an internal dividing wall still standing. The wheel-pit alongside the south-east wall, about 2.67 metres wide, would have housed a breastshot suspension wheel, a type where water strikes the wheel at roughly mid-height, giving good mechanical advantage on rivers without dramatic falls. The bearing stone for the wheel's axle remains in place in the mill wall, and a brick-arched opening just over a metre wide marks the point where power was transferred into the building's interior. Three large foundation stones for the mill machinery survive inside, and a conglomerate runner millstone, the upper stone in a grinding pair, lies on the ground to the south. The millrace that fed the wheel is now dry, drawn off from the Awbeg roughly 500 metres to the east. On the Ordnance Survey six-inch map of 1842, the site appears as Ballyhay Mill, suggesting a working identity distinct from the townland name of Rathmorgan by which it is known today. On higher ground to the north-east, the remains of two small rectangular single-storey buildings may have served the mill in some capacity, though their precise function is unclear.