Boyne, river, c.70 mi (110 km) long, rising in the Bog of Allen, Co. Kildare, E Republic of Ireland, and flowing NE through Co. Meath, past Trim, to the Irish Sea near Drogheda. Salmon is caught in the river. In the battle of the Boyne (July, 1690) near Drogheda, the armies of King William III defeated the Catholic James II, who fled to France. The victory is commemorated annually on July 12 by Protestants in Northern Ireland.