The River Shannon is 360 km (224 miles) and is by far the longest river in Ireland and is named after the Celtic goddess, Sionna. Its source is the Culicagh Mountain in co. Cavan and it then flows through 11 counties towards the Shannon Estuary in Limerick. The river is relatively slow-moving as it only falls 60 feet in the first 150 miles of its course. It is the natural divide for 3 of Ireland’s provinces with Connacht to the west and Leinster and Munster to the east and south. The river has 3 major lakes – Lough Allen, Lough Derg, and Lough Ree- which are also connected by many canals including the Royal Canal and the Grand Canal. For thousands of years, it has always been strategically important and had its own set of Martello Towers and defenses built at the start of the 1800s for the threat of a Napoleonic Invasion. The largest towns on the river are Limerick and Athlone. Legend has it that a monster called Cata lives in the river!