The Irish Sea is connected to the North Atlantic at both its northern and southern ends. To the north, the connection is through the North Channel between Scotland and Northern Ireland and the Malin Sea. The southern end is linked to the Atlantic through the St George's Channel between Ireland and Pembrokeshire, and the Celtic Sea. It is composed of a deeper channel about 190 miles (310 km) long and 20–30 miles (32–48 km) wide on its western side and shallower bays to the east. The western channel's depth ranges from 80 metres (260 ft) up to 275 m (902 ft) in the Beaufort's Dyke in the North Channel.
Cardigan Bay in the south, and the waters to the east of the Isle of Man, are less than 50 m (160 ft) deep. With a total water volume of 2,430 km3 (580 cu mi) and a surface area of 47,000 km2 (18,000 sq mi), 80% is to the west of the Isle of Man. The largest sandbanks are the Bahama and King William Banks to the east and north of the Isle of Man and the Kish Bank, Codling Bank, Arklow Bank and Blackwater Bank near the coast of Ireland. The Irish Sea, at its greatest width, is 120 miles (190 km) and narrows to 47 miles (76 km).
The International Hydrographic Organization defines the limits of the Irish Sea (with St George's Channel) as follows,
- On the North. The Southern limit of the Inner Seas off the West Coast of Scotland, defined as a line joining the South extreme of the Mull of Galloway (54°38'N) in Scotland and Ballyquintin Point (54°20'N) in Northern Ireland.
- On the South. A line joining St. David's Head in Wales (51°54′N 5°19′W / 51°54′N 5°19′W / 51.900; -5.317) to Carnsore Point in Ireland (52°10′N 6°22′W / 52°10′N 6°22′W / 52.167; -6.367).
The Irish Sea has undergone a series of dramatic changes over the last 20,000 years as the last glacial period ended and was replaced by warmer conditions. At the height of the glaciation, the central part of the modern sea was probably a long freshwater lake. As the ice retreated 10,000 years ago, the lake reconnected to the sea.