inlet on the north coast of Ireland between the Inishowen Peninsula (mainly County Donegal, Ireland) to the west and the district councils of Limavady and Londonderry (until 1973 in County Londonderry), Northern Ireland, to the east and southeast. The lough is about 16 miles (26 km) long and varies in breadth from 1 to 10 miles (1.6 to 16 km). The narrowest points are at the southwestern end, where the River Foyle enters the lough, and at the northeastern end, opposite Magilligan Point. There are historic settlements on the western shore of the lough, which has been an important fishing area and passageway since early times.