Unlike the more familiar vertical waterwheel, the horizontal mill, sometimes called a Norse or tub mill, works on a simpler principle: water is directed through a chute at an angle onto a wheel lying flat in the stream, which then drives a millstone directly above it without any system of gears. This simplicity made the design cheap to build and easy to maintain, and for centuries it served rural communities across Ireland who needed to grind small quantities of grain close to home.

Horizontal mills of this kind were already ancient technology by the medieval period, and Ireland has a particularly strong association with them; some of the earliest dated examples in Europe have been found here, with certain excavated sites going back to the seventh and eighth centuries. They were a fixture of the pre-industrial rural economy, typically built on fast-running streams too modest to power a larger vertical mill. Most were eventually abandoned as commercial milling took over in the eighteenth and nineteenth centuries, and the physical remains, usually low stone footings and a carved millstone or two, are easy to overlook in the landscape. The Ballyguin example in Mayo represents this once-widespread but now largely forgotten form of local industry.