The Irish Sea (Irish: Muir Éireann or Muir Meann,Scottish Gaelic: Muir Èireann or Muir Mhanainn,Manx: Mooir Vannin or Y Keayn Yernagh,Welsh: Môr Iwerddon, Scots: Erse Sea) separates the islands of Ireland and Great Britain. It is connected to the Celtic Sea in the south by St George's Channel, and to the Atlantic Ocean in the north by the North Channel. Anglesey is the largest island within the Irish Sea, followed by the Isle of Man. The sea is occasionally, but rarely, referred to as the Manx Sea.

The sea is of significant economic importance to regional trade, shipping and transport, fishing, and power generation in the form of wind power and nuclear plants. Annual traffic between Great Britain and Ireland amounts to over 12 million passengers and 17 million tonnes of traded goods.

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 Scottish Seas [or 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 Ireland"].