The longest river in Ireland is the River Shannon, which is approximately 360 miles long. It is the second longest river in the British Isles and flows through the Republic of Ireland and Northern Ireland. The river begins in County Cavan and flows southwards to the Atlantic Ocean. Along its course, it passes through several lakes, including Lough Allen, Lough Ree, and Lough Derg. The River Shannon is an important source of hydroelectric power and is also used for recreational activities such as fishing and boating.