Shannon River, the longest and the most important river in Ireland. It begins in the Iron Mountains near the boundary of Northern Ireland and flows southward and westward for about 230 miles (370 km) to the Atlantic Ocean. The Shannon passes through several lakes, the largest of which are Lough Ree and Lough Derg. West of Limerick, the main city on the Shannon, the river broadens into an estuary, which is navigable by oceangoing ships. There is a hydroelectric station between Lough Derg and Limerick.