Nobody today can say exactly where it stood. Its precise location has never been identified, which gives the whole site an unusual quality: the mill is simultaneously documented and lost, present in the historical record but absent from the landscape in any clearly legible form. What may survive is a curving linear earthwork visible on aerial photographs to the south of the castle, possibly the remnant of a mill race, the channel that would have directed water from the river to power the mill's wheel. Without that controlled flow of water, no mill could function, and such earthworks were as essential to the operation as the millstone itself.

The paper trail for the mill leads back to the Civil Survey of 1655, a wide-ranging Cromwellian-era land assessment that recorded the state of Irish property as it had been in 1640, just before the upheavals of the 1641 rebellion. According to that survey, a man named Mortagh Mc Bryne, described as an 'Irish Papist of Longford', was in possession of 'a mill and a Castle in repaire' at Coonagh in 1640. The phrasing is worth pausing over: the castle was noted as being in repair, a detail that implies active habitation and upkeep at a time when many such structures were falling into neglect. The mill's inclusion in the same entry suggests it was functioning as part of a working estate. Adding further texture to the site, a deserted medieval settlement appears to have existed in the fields to the east of the Cahernahallia River, near Castletown Church, pointing to a broader community that would have depended on precisely this kind of milling infrastructure.

For anyone drawn to this corner of County Limerick, the experience is less about seeing something intact and more about reading a landscape for faint traces. The area around the Cahernahallia River and Coonagh Castle rewards patient attention, particularly from someone willing to cross-reference aerial photographs with what is visible on the ground. The curving earthwork to the south of the castle is the most tangible candidate for the mill race, though it takes some imagination to reconstruct what the working complex would have looked like. Castletown Church and the fields to the east of the river round out a cluster of related monuments that together suggest a once-busy rural settlement, now largely absorbed back into the quiet County Limerick countryside.