Irish Sea, arm of the Atlantic Ocean, c.40,000 sq mi (103,600 sq km), 130 mi (209 km) long and up to c.140 mi (230 km) wide, lying between Ireland and Great Britain. It is connected with the Atlantic by the North Channel and (on the south) by St. George's Channel. Ireland is on its west shore, Scotland, England, and Wales on the east. The principal islands in the sea are the Isle of Man, Anglesey, and Holyhead. The chief ports are Dublin, Liverpool, Manchester, Fleetwood, and Dún Laoghaire.