#[2] can you please explain?
Discussion
Northern Ireland is a devolved government within the UK. Its a form of federalism but it differs in that the "states" are self governing but they contained within another country.
It's a special statutory instrument that had to be made on the fly in the wake of an armed rebellion in 1916, that's why its different to the other countries you mentioned. In 1920 , the British crown had to satisfy 3 different groups, Irish nationalists , Northern Irish unionists and its own self interest. This "devolved" government is the result of that. Northern Ireland is a self governing (more or less, but not exactly, it couldn't survive without the UK economically) territory within the UK.
There are however Ulster nationalists who believe that Northern Ireland should be its own country.
Coincidently , those other countries within the UK you mentioned were granted this "devolved" status much later, while still retaining the "country" status.