On Tuesday last Captain Kingsley, who is on a visit with his father, Capt Kingsley of Knigh Cottage, swam across Lough Derg, in the Shannon, from Dromineer Bay to Williamstown, a distance of five miles, and was not in the least fatigued at the end of his journey. Such a feat which has not been before performed within the memory of any person now living.
Tipperary Free Press 23 August 1864 quoting Nenagh Guardian
Irish waterways history 
Has anybody swam it since then ?
I don’t know. bjg