The United Kingdom of Great Britain and Northern Ireland is a former state which existed in that form from 1921 to 1993. It came to an end as a result of the deal which held the coalition government of 1988-93 together, made between the Labour Party and the Federalist Alliance.
The agreement made between the parties was that the Federalist Alliance parties would support the Labour Party in Parliament and local elections provided that at the end of that Parliamentary term the nations represented would become independent of the United Kingdom and that the regionalist parties would also get their own tier of government akin to a state government like West Germany.
The territory previously referred to by the British government as Ulster became part of the Republic of Ireland, as the Irish Constitution itself had always claimed. This was more feasible than it had been in previous decades for two reasons. Firstly, religious sensibilities had considerably declined by that time and most of the people, of whatever affiliation, no longer felt particularly that they were Catholic or Protestant, although the political history remained. Secondly, whereas Republican sentiments had long been supported by outside interests, the same was not true of the Unionists and there was little violent resistance although there was quite a lot of emigration in the next couple of years.
Scotland, Wales, Cornwall, England, Forvik and Orkneyjar each became entirely independent nations. They remained Crown Dependencies like the Isle of Man and the Channel Isles, and being part of the Shengen Area continued in a customs union, but apart from that had no more connection with each other than the UK had previously had with the Republic of Ireland.
Moreover, a new tier of government arose within England, so that there were six regions with state-like control over their own affairs.