What came up was modest in scale but remarkable in age: a horizontal-wheeled mill, a type in which the wheel lies flat in the water and drives the millstone directly above without the need for gearing, a simpler and older design than the more familiar vertical waterwheel. The structure measured just 2.7 metres in length, and five timber planks of varying sizes were recovered from it, the longest running to 2.43 metres, around 28 centimetres wide and 15 centimetres thick.

What makes the Keelaraheen mill genuinely arresting is its date. The timbers were subjected to dendrochronology, a method of dating wood by matching the pattern of its annual growth rings against a long-term reference chronology, and the analysis placed their felling at around A.D. 843. That puts this mill firmly in the early medieval period, contemporary with monastic Ireland and the early phases of Viking activity along the island's coasts and rivers. Horizontal mills of this type were widespread across early medieval Ireland, and several examples with comparably ancient dates have been identified through dendrochronological work, but each one adds something to the picture of how rural communities managed grain processing over a millennium ago. The scholarship on this particular find was published by Colin Rynne in 1988, drawing on dendrochronological work carried out by M. G. L. Baillie.