Lark: Personal knowledge. I used to fly-fish there, back in the day. Mind you, this was when Ennell was suffering from the worst eutrophication problem of any waterway in western Europe, when my time would as profitably have been employed flogging a wet runway.
I understand it's a bit better these days, as a result of the farmers on the Westmeath side no longer being allowed to spray liquid pigs**t on their fields within a defined radius of the Brosna.