The River Blackwater in Fermoy, Ireland (North of Cork), is frozen. According to local historian Christy Roche, the last time that happened was the winter of 1601 after the Battle of Kinsale. Sea temperatures are not responsible. The seas in the area are at about 10 C.
In 1963 there was some ice on the river during heavy snowfall, but on that occassion the river froze only half way across. It is reported that the ice is quite thick on the river.