It is an easy detail to overlook, yet it quietly connects the town's most recognisable medieval structure to a milling operation that was already old when cartographers first recorded it.

The earliest documentary evidence comes from the Down Survey of 1655 to 1656, the ambitious mapping project commissioned by the Cromwellian administration to catalogue confiscated Irish land. That survey marks a mill at the northern end of Swords village, and the corn mill shown on the first edition Ordnance Survey six-inch map, produced in the nineteenth century, is thought to occupy the same general site. A painting by George Holmes, made around 1790, adds a visual dimension: it depicts a mill wheel positioned to the south of what cartographic sources call Mill Bridge, with a weir crossing the river at that point. A weir is a low dam built across a river to raise the water level and control its flow, allowing a reliable head of water to be directed into a millrace. Photographs held in the National Library of Ireland collections, dating to around 1900, show both the wheel and the weir still in place at that time, offering a rare glimpse of the working mill in its final decades.

The castle itself, a large medieval enclosure associated with the Archbishops of Dublin, is open to visitors and managed by the Office of Public Works. Walking the western perimeter, it is possible to trace the line of the millrace as it follows the curtain wall. The Mill Bridge, named for its proximity to the old mill site, remains a useful orientation point for anyone trying to reconstruct where the wheel once sat in relation to the river. The National Library's photographic collections, accessible online, are worth consulting before a visit; seeing the c.1900 images of the wheel and weir against the actual landscape gives the surviving earthwork features a good deal more meaning.