The River Shannon is the longest river in Ireland at 360.5 kilometres (224 miles). Its official source is in County Cavan on the slopes of the Cuilcagh Mountain in a bubbling spring known as “The Shannon Pot”. The river develops into three lakes along its course, Lough Allen, Lough Ree and Lough Derg. Lough Derg is the largest of these three lakes. The Shannon goes through six Counties before entering the Atlantic Ocean at the Shannon Estuary.