Irish Sea, (ancient Oceanus Hibernicus), arm of the Atlantic Ocean, between Ireland on the west and Great Britain on the east, and covering an area of about 104,000 sq km (40,000 sq mi). The sea is connected to the Atlantic by the North Channel, on the north-west, and by St George's Channel, on the south. The main islands are the Isle of Man, in the centre of the sea, and Anglesey, adjacent to the Welsh coast. The sea was formed by rifting and subsidence during the Tertiary period, and has a maximum depth of 175 m (576 ft). Liverpool and Dublin are the main ports on the sea.