This type of mill, sometimes called a Norse or tub mill, is one of the earliest milling technologies known in Ireland. Unlike the familiar vertical wheel, a horizontal-wheeled mill uses a wheel or paddle assembly mounted flat beneath the mill-house, turned directly by a jet of water channelled through a wooden flume or head-race. The design is simple, requires no complex gearing, and was widespread in early medieval Ireland, particularly on smaller streams where water flow was modest but reliable.

The discovery at Treanacally yielded fifteen timbers in total, though most were damaged or fragmentary by the time they came to light. Six were sufficiently intact to be removed for preservation. Two large solid beams, each approaching three metres in length and roughly forty centimetres wide, bear the mortise-and-tenon joinery typical of early timber construction and are thought to have formed part of the mill-house walls or roof. Three thinner planks, one fitted with a tenon on its underside and another with tenons at both ends and an angled connecting fragment, are interpreted as floor timbers. The sixth and perhaps most revealing piece is a hollowed-out timber with raised sides that narrows toward one end, terminating in a tenon joint; this is a section of the head-race, the channel that directed water under pressure toward the wheel. Two millstones were also recovered. The larger, with a smooth face and a central perforation ten centimetres across, is almost certainly the upper stone of a grinding pair, the one that rotated above the fixed lower stone to crack and crush grain. The smaller stone, only thirty centimetres in diameter, has an irregular outline and a worn underside, suggesting long use despite its modest scale. One timber remained in place at the stream edge, probably part of the original mill-house wall, left where the digger had found it.