Shannon, principal river of the Republic of Ireland and longest (c.240 mi/390 km) in the British Isles. It rises near Cuilcagh Mt., NW Co. Cavan, and flows S through the Central Plain into Co. Limerick, where it turns west in a broad estuary (c.70 mi/110 km) to the Atlantic Ocean between Loop Head and Kerry Head. Loughs Allen, Boderg, Ree, and Dreg are expansions of the river. The Shannon with its many tributaries drains a region of farmland and peat bogs. Towns along the Shannon include Carrick-on-Shannon, Athlone, and Limerick. The river is connected with E Ireland by the Royal Canal and the Grand Canal; large vessels, however, cannot ascend the river above Foynes. Limerick is the head of estuary navigation. An important hydroelectric plant was built between Lough Derg and Limerick. The fisheries of the river are valuable. Shannon Airport Industrial Estate (opened 1945), a duty-free international terminal on the river, 15 mi (24 km) W of Limerick, has been developed by the government into an industrial center.