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Nendrum Monastery Tidal Mill: 620 AD
The use of the ocean tides for energy is not new. In 620 AD in Northern Ireland at the Nendrum Monastery Mill a tidal mill was built. It used a millpond that was filled with seawater at high tide, which was released through a sluice and fed along a channel to the mill's horizontal wheel as the tide receded.
Nendrum Tidal Mill Illustration
On the shore close to the NE side of the Nendrum Monastery Site a substantial stone wall runs parallel to the shoreline for about 120 metres at a distance of about 20 metres out from the high tide line.
Investigation of the so called fish pond in the summer of 1999 led to the discovery of a stone built tide mill and evidence of its earlier evolution. Calculations estimate power at the millstone is seven eighths of a horse power at the start of discharge from a spring tide which reduces to less than half a horse power as the mill pond is emptied. This tidal mill could have ground up to 1 ton of grain a month.

sources: http://www.nendrum.utvinternet.com/tmill