The Nendrum Monastery mill was a tide mill on an island in Strangford Lough now in Northern Ireland. It is the earliest excavated tide mill, dating from 787 AD. Its millstones are 830mm in diameter and the horizontal wheel is estimated to have developed 7/8HP at its peak. Remains of an earlier mill dated at 619 AD were also found.
References
Nendrum Monastery mill Wikipedia(Text) CC BY-SA