Arm of the North Atlantic Ocean that separates Ireland from Great Britain. The Irish Sea is bounded by Scotland on the north, England on the east, Wales on the south, and Ireland on the west. The sea is connected with the Atlantic by the North Channel between Northern Ireland and Scotland and by St. George's Channel between the southeastern tip of Ireland and southwestern Wales. The sea is about 130 miles (210 km) long and 150 miles (240 km) wide. Its total area is approximately 40,000 square miles (100,000 square km). Its greatest depth measures about 576 feet (175 m) at the Mull of Galloway, near the sea's junction with the North Channel. In classical times the Irish Sea was known as Oceanus Hibernicus.