The Irish Sea separates the islands of Ireland and Great Britain. It is connected to the Atlantic Ocean (the Celtic Sea) by St George's Channel between Ireland and Wales in the South and by the North Channel between Ireland and Scotland in the North.

The Isle of Man lies in the middle of the Irish Sea.

Ferry connections between Britain to Ireland via the Irish Sea include the routes from Swansea to Cork, Fishguard and Pembroke to Rosslare, Holyhead to Dún Laoghaire, Stranraer to Belfast and Larne, and Cairnryan to Larne. There is also a connection between Liverpool and Belfast via the Isle of Man.

The Irish Sea Forum is an environmental forum concerned with the Irish Sea.

The Irish Sea has been subject to heavy radioactive pollution by the first British weapons grade ²³⁹Pu nuclear production plant and power station at Windscale. An estimated 250 kg of plutonium have been deposited in marine sediments during the first decades of production.

The Irish Sea has undergone a series of dramatic changes over the last 20 000 years as the last ice age ended and was replaced by warmer conditions. At the height of the ice age the central part of the modern sea was probably a long freshwater lake. As the ice retreated 10 000 years ago the lake re-connected to the sea, becoming brackish and then fully saline once again.