The River Shannon is the principal river on the island of Ireland. It rises in County Cavan in the Cuilcagh Mountains and flows in an arc, first north, then west, southwest, and finally west again. It passes through loughs Allen, Boderg, Ree, and Derg before reaching Limerick, at which point it widens into an estuary. It enters the Atlantic Ocean between Loop Head and Kerry Head after a total course of about 250 miles.
The principal tributaries are the Boyle, the Suck, and the Fergus from the right, and the Inny, Brosna, Mulkear, Maigne, and Deel from the left. The Shannon is tidal as far as Limerick. It is joined by the Royal Canal at Longford and by the Grand Canal at Offaly.