Forecasts for the marine community were first issued in these parts in the early 1860s, prompted by a number of spectacular shipwrecks during the previous decade which highlighted the potential benefits to seafarers of advance warnings of severe weather.
The coasts of Britain, Ireland and the North of France were divided for the purpose into six "districts". They comprised West-central, Southwest and South-east England, the East Coast of England, and the coasts of Scotland and Ireland. A daily "two-day opinion" was provided for each district, and promulgated to the major ports by the newly invented electric telegraph.
However, marine forecasts have changed over the years. The districts now are gone - or at least they have been rearranged. Nowadays, the sea area forecast you hear from RTE refers to a strip of water about 35 miles around the coast of Ireland. This strip is subdivided by referring to headlands around the coast, identifying in detail the zone to which the forecast refers.
The equivalent in Britain is called the Shipping Forecast. The British authorities have a formal responsibility to provide weather information to shipping far out into the Atlantic. The scheme was designed in the 1920s, and now divides the zone of interest into 31 "sea areas" of varying sizes; forecasts are provided for each of them in a clockwise sequence around the islands of Britain and Ireland, and covering a roughly triangular region bounded by Iceland, Norway and Portugal.
Most of the sea areas are named after some geographical landmark in the locality. Lundy, Fair Isle, Faeroes, Fastnet, Rockall and Hebrides, for example, are all called after islands or rocks within their boundary; similarly North and South Utsire are named after an island near the Norwegian coast beside Stavanger. The sea areas along the east coast of England and Scotland are named from rivers flowing into them - Cromarty, Forth, Tyne, Thames, and so on; Shannon, too, comes into this category. Biscay and German Bight, on the other hand, are indentations in the coastlines.
Sea areas like Dogger and Bailey are called after banks or shallows in the sea, and yet others are named after headlands, like Portland and Finisterre. Trafalgar, the most southerly of the sea areas, is also named after a headland, but has the distinction of not enveloping its eponym; one presumes that the historical associations of Cape Trafalgar on the south coast of Spain, some 100 miles outside the boundary, were too much for the patriotic designers of the system to resist.