For years it appeared on Ordnance Survey mapping as a mill of apparent antiquity, and a 17th-century Down Survey parish map of Kilmurry does indeed mark a watermill in this vicinity, inviting the assumption that the two referred to the same structure. Archaeological investigation eventually closed that gap, finding not the continuity that had been assumed, but a straightforward absence.

The Down Survey, a massive land-mapping project carried out in the 1650s under William Petty, recorded the landscape of Ireland in extraordinary detail for its time, and the Kilmurry parish map, held as NLI MS 718, shows two watermills near Ballysimon alongside the castle recorded at the same location. When archaeologist Tracy Collins excavated the site in 1998 (licence 98E0607), the building under investigation was a modest structure of roughly coursed rubble limestone, measuring approximately five metres by seven metres. Because the site had been reused as a creamery and later as a farmyard, the only workable position for a trench was inside the building itself, in its south-east corner. What emerged from a careful hand-excavation was a cobbled floor set in white mortar, walls founded directly on red boulder clay, and a narrow plinth running along the base of the east wall. That plinth, the excavators reasoned, had been added to stabilise the wall and protect it against water undermining, since it was the wall that would originally have carried the overshot wheel, the type in which water is delivered from above to buckets on the wheel's rim, driving it by weight rather than flow. No earlier structure was found beneath it, no artefacts of archaeological significance turned up, and the conclusion was unambiguous: the surviving mill is 19th century in date and has no demonstrable connection to the watermill marked on the 17th-century survey.

The remains are slight and the site has had several practical lives since its milling days. The building fabric is described as unstable in places, so any visit warrants caution around the walls. For those interested in the Down Survey map that prompted the investigation in the first place, NLI MS 718 is accessible through the National Library of Ireland and shows both the mill sites and Ballysimon Castle in their 1657 context, offering a reminder of how confidently old maps can lead researchers in one direction while the ground quietly tells a different story.