The River Shannon (Irish: Abha na Sionainne / an tSionainn / an tSionna) is the longest river in Ireland at 360.5 km (224 miles).
It divides the west of Ireland (principally the province of Connacht) from the east and south (Leinster and most of Munster). County Clare, being west of the Shannon but part of the province of Munster, is the major exception. The river represents a major physical barrier between east and west, with fewer than thirty crossing-points between Limerick city in the south and the village of Dowra in the north.
The river is named after Sionna, a Celtic goddess.
The Shannon has been an important waterway since antiquity, having first been mapped by the Graeco-Egyptian geographer Ptolemy. The river flows generally southward from the Shannon Pot in County Cavan before turning west and emptying into the Atlantic Ocean through the 113 km (70 mi) long Shannon Estuary. Limerick city stands at the point where the river water meets the sea water of the estuary. The Shannon is unaffected by sea tides east of Limerick.
The river began flowing along its present course after the end of the last Ice Age. Vikings settled in the region in 10th century and used the river to raid the rich monasteries deep inland. In 937 the Limerick Vikings clashed with those of Dublin on Lough Ree and were defeated.
In the 17th century, the Shannon was of major strategic importance in military campaign in Ireland, as it formed a physical boundary between the east and west of the country. In the Irish Confederate Wars of 1641-53, the Irish retreated behind the Shannon in 1650 and held out for two further years against English Parliamentarian forces. In preparing a land settlement, or plantation after his conquest of Ireland Oliver Cromwell reputedly said the remaining Irish landowners would go to 'Hell or Connacht', referring to their choice of forced migration west across the river Shannon, or death, thus freeing up the eastern landholdings for the incoming English settlers.
In the Williamite war in Ireland (1689–91), the Jacobites also retreated behind the Shannon after their defeat at the Battle of the Boyne in 1690. Athlone and Limerick, cities commanding bridges over the river, saw bloody sieges. (See Sieges of Limerick and Siege of Athlone).
As late as 1916, the leaders of the Easter Rising planned to have their forces in the west 'hold the line of the Shannon'. However, in the event, the rebels were neither well enough armed nor equipped to attempt such an ambitious policy.