The oak beam on which it is built, at least 3.3 metres long and cut with two mortices, was pulled from the stream bed around 1980. It is now doing unremarkable duty as a farm gatepost, roughly 350 metres from where it lay for over a thousand years. Those mortices are the giveaway: this timber was once part of a horizontal-wheeled water mill, a type of early medieval mill in which a wheel lying flat in the current drove a millstone directly above it, without the need for gearing. Simple in principle, these mills were widespread in early Christian Ireland, and this particular one dates to a remarkably precise moment in the past.

Dendrochronology, the science of dating timber by its annual growth rings, placed the felling of the oak at AD 873, give or take nine years. That analysis was carried out by M. Baillie at the Palaeoecology Laboratory in Queen's University Belfast, one of the leading centres for this kind of work on Irish archaeological wood. The date puts the mill firmly in the ninth century, a period of considerable activity and disruption across Ireland, and gives it an age that most above-ground monuments in the country cannot match. Other timbers from the same structure are believed to remain in the stream bed, which sits on the north bank of the small east-west watercourse before it meets the Kayle about 350 metres to the west. Close by, roughly ten metres to the northwest, lies a burnt mound, one of the low, kidney-shaped heaps of fire-cracked stone associated with ancient cooking or industrial processes, suggesting the immediate area saw sustained use well before and around the mill's working life.