Recent archaeological discoveries of early medieval horizontal water-wheels in Ireland have shed important new light on the origins and technical development of the horizontal water-wheel in Asia and Europe. As will be argued below, based on this evidence, a number of general patterns are beginning to emerge. In the first of these it is clear that regional variations on basic types had already developed across the island of Ireland from at least the seventh century onwards. Indeed, a number of these also appear to have been designed to accommodate seasonal water flows. The recent Irish evidence is also compared with what is currently known from medieval written sources from Europe. Together, these demonstrate clear continuity in the use of certain forms of horizontal water-wheel from the early medieval period up to very recent times.