It was found entirely by accident in 1971, when drainage work broke through to a scatter of waterlogged timber, and what emerged was the fragmentary but legible skeleton of a horizontal-wheeled mill dating to around AD 600.

Horizontal mills, sometimes called Norse mills though they predate Scandinavian influence in Ireland, work on a simple principle: water is directed through a chute onto a wheel mounted horizontally beneath the millstone, turning the stone directly without the need for gearing. The Knocknacarragh find included roughly twenty pieces of timber, among them parts of the building frame and uprights, two water chutes, and a bung used to control water flow. A radiocarbon date taken from one of the timber samples placed the mill at approximately AD 600, situating it in the early medieval period, when horizontal mills of this type were spreading across Ireland and becoming central to monastic and rural economies. The timber was preserved because it had been sealed in waterlogged ground, the same anaerobic conditions that make wetland archaeology so unexpectedly productive.