The fall of the Shannon is concentrated between Killaloe, at the bottom of Lough Derg, and Limerick, at the head of the Shannon Estuary. It is that fall, of almost 100 feeet, that made possible the construction of the hydroelectric power station at Ardnacrusha in the 1920s. Its designers were not the first people to realise the usefulness of the water-power of the Shannon in that area, but they were almost the first to use it: there were few mills between Killaloe and Plassey, and that at O’Briensbridge used water from the Bridgetown direction rather than from the Shannon.
There were, however, two bleach mills, one at Doonass on the County Clare side of the Shannon and one at Castleconnell on the Co Limerick side. The Doonass mill seems to have been set up, around 1760, by Hercules Browning or Brownriggs. There is little trace of the mill itself, but its intake and outlet canals are still to be seen. The really interesting thing is that there are two outlets: the shorter returns to the Shannon almost immediately below the mill while the longer runs for almost half a mile, behind a hill, before it rejoins the river.
I don’t know why it has two outlets. It is possible that the system catered for much higher water levels with greater variation between summer and winter. It is conceivable that the longer arm might have been used to carry the produce of the mill downstream, although I have no evidence for that and I’m not sure where the goods would have gone after the outlet rejoined the river. The watercourse is referred to locally as Conway’s Canal, but I don’t think that is evidence that it ever carried anything.
Anyway, here are maps, photos and as much background information as I could find. Comments, suggestions and explanations will be welcome.
Irish waterways history 