Irish Sea, a body of water that lies between the island of Great Britain on the east and the island of Ireland on the west. The North Channel on the north and St. George's Channel on the south connect the Irish Sea with the Atlantic Ocean. The sea is about 160 miles (260 km) long (north to south) and up to 120 miles (190 km) wide. The largest islands are the Isle of Man and Anglesey. Liverpool and Dublin are principal ports.