The first river of Ireland, and largest in the British Islands, rises in the Cuilcagh Mountains, Co. Cavan; flows in a south-westerly direction through Loughs Allen, Ree, and Derg, besides forming several lough expansions, to Limerick, whence it turns due W., and opens out on the Atlantic in a wide estuary between Kerry (S.) and Clare (N.); has an entire course of 254 m., and is navigable to Lough Allen, a distance of 213 m.