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The River Shannon ( altenatively Sionna), Ireland's longest river, divides the West of Ireland (mostly the province of Connacht) from the east and south (Leinster and most of Munster). The river has been an important waterway since antiquity. First mapped by Ptolemy, the river is 386 km (240 mi) long, and thus it exceeds the length of all other rivers in Ireland. The river flows generally south from the Shannon Pot in County Cavan before turning west and emptying into the Atlantic Ocean through the 113 km (70 mi) long Shannon Estuary. Limerick city stands watch at the point where the river water meets the sea water of the estuary. Only east of Limerick is the river no longer affected by the tides.