The mill at Bunnadober in County Mayo is something quite different. It is one of only a handful of surviving examples of a horizontal-wheeled grain mill, a design in which the wheel lies flat and water strikes it from below or the side to turn a vertical shaft directly. This arrangement was so thoroughly superseded in Ireland that it had largely fallen out of use by the mid-seventeenth century, which makes the Bunnadober mill not merely old but genuinely anomalous, a working relic of a milling tradition that almost entirely vanished.

The building itself is a modest, three-bay single-storey structure with a half-attic, constructed from rubble limestone with battered walls, meaning the walls slope slightly inward as they rise, a technique that adds stability. The corner stones are tooled hammered limestone flush quoins, one of which carries a benchmark inscription, the kind of surveyor's mark cut into masonry as a fixed reference point for measuring elevation. The roof retains its pitched slate finish with clay ridge tiles, and the cast-iron rainwater goods are still in place. The mill appears by name on the first edition Ordnance Survey six-inch map, was recorded as operational in 1916, and continued working until 1980, an extraordinary span for a machine of such archaic design. It was conserved in 2001 and is now a national monument in State care. Inside, the drive shaft mechanism and horizontal mill wheel survive, giving a rare opportunity to understand how this older form of milling actually functioned before the vertical wheel became universal.