Which sea lies off southern Ireland and near Cornwall and Brittany?

The sea off southern Ireland and near Cornwall and Brittany is the Celtic Sea.

The Celtic Sea is a region of the Atlantic Ocean north of the Bay of Biscay. It lies south of Ireland and reaches toward Wales, Cornwall, Devon and Brittany. Saint George’s Channel, the Bristol Channel and the English Channel form parts of its northern and eastern limits, while the continental shelf marks much of its open southern and western edge.

The name reflects the Celtic cultural connections of the surrounding lands, including Ireland, Wales, Cornwall and Brittany. Modern geographical definitions were formalised for nautical use, with the UK Hydrographer of the Navy approving a chart definition in 1974. The International Hydrographic Organization also publishes limits for the sea.

The Celtic Sea is not a completely enclosed basin: toward the south and west it opens into the Atlantic. The Iroise Sea off Brittany is included within it, and the Isles of Scilly sit near its northeastern edge. A frequent mix-up is treating it as the same water as the Irish Sea, which lies north of Ireland and is separated from it by channels and headlands.