Northern Ireland, integral part of the United Kingdom of Great Britain and Northern Ireland, situated in the northeastern portion of the island of Ireland. Northern Ireland is bounded on the north and northeast by the North Channel, on the southeast by the Irish Sea, and on the south and west by the Republic of Ireland. It includes Rathlin Island in the North Channel and several smaller offshore islands. Northern Ireland is also known as Ulster, because it comprises six of the nine counties that constituted the former province of Ulster. The total area of Northern Ireland is 14,148 sq km (5463 sq mi).