Which river is the longest river on the island of Ireland?

The River Shannon is conventionally recognised as the longest river on the island of Ireland, measuring about 360.5 kilometres from its traditional source to the Shannon Estuary.

It rises at Shannon Pot in County Cavan and flows generally south before turning west. Along its course, it passes through or connects with major lakes including Lough Allen, Lough Ree and Lough Derg, then reaches the Atlantic through the Shannon Estuary near Limerick.

The Shannon is also Ireland’s largest river by average flow and drains a basin covering roughly one-fifth of the island. Length comparisons can become complicated because some studies trace the river through longer tributary headwaters, producing a longer Boyle–Shannon or Owenmore–Shannon route. Those are measurement conventions; the standard named river and quiz answer remains the River Shannon. The River Liffey, by contrast, is the river associated with Dublin.