Mourne (named after the Múrna) is a barony in County Down, Northern Ireland. It lies in the south-east of the county, with the Irish Sea to its east. It is bordered by two other baronies: Iveagh Upper, Lower Half and Iveagh Upper, Upper Half to the north and west. Mourne gets its name from the Múrna (Old Irish: Mughdorna), a people who hail from a territory of the same name in modern County Monaghan.