During land reclamation work at Fox's Castle in County Waterford around 1970, labourers turned up something unexpected beneath the soil of the Tay River's eastern bank: a collection of worked oak timbers, some bearing mortises, the carefully cut rectangular slots used in traditional joinery to receive a corresponding tenon. The timbers are the surviving traces of a horizontal-wheeled water mill, a type of early medieval milling technology in which the wheel lies flat beneath the millstone, oriented horizontally rather than vertically as most people picture a water mill today. Water was directed through a narrow chute or flume onto the paddles of the horizontal wheel, which rotated directly on a vertical shaft connected to the millstone above. Simple in construction and requiring no complex gearing, these mills were once widespread across Ireland and are among the earliest forms of mechanised grain processing in the country.

The Tay River, running south to north through this part of Waterford, would have provided the reliable flow such a mill required. Oak was the preferred timber for medieval mill structures, valued for its durability even in waterlogged conditions, which explains why these particular pieces survived long enough to be recovered. Mortised timbers of this kind are a known feature of early Irish mill construction, and comparable remains have been found at other waterlogged sites across the country, occasionally alongside wooden mill wheels or fragments of the chute apparatus. The precise date of the Fox's Castle mill is not recorded, but horizontal-wheeled mills in Ireland are generally associated with the early medieval period, roughly the sixth to the twelfth centuries, when they were a common feature of monastic and secular settlements alike.