“known colloquially as Ulster” means not a legal term.
But Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone are.
And those names have been around for quite some time.
Judea and Samaria or part of the Occupied Territories?
A name gives identity, existence and ownership.
I apologize for being lazy.
"The Plantation of Ulster (Irish: Plandáil Uladh) was the organised colonisation (or plantation) of Ulster by people from Great Britain (especially Presbyterians from Scotland). Private plantation by wealthy landowners began in 1606...
"Attorney-General for Ireland in 1609, Sir John Davies [explained]: 'A barbarous country must be first broken by a war before it will be capable of good government; and when it is fully subdued and conquered, if it be not well planted and governed after the conquest, it will eftsoons return to the former barbarism.'
"In the 19th century, Ulster had the only large-scale industrialisation and became the most prosperous province on the island. In the latter part of the century, ...Belfast became famous in this period for its huge dockyards and shipbuilding.
"Northern Ireland's current politics originate from these late 19th century disputes over Home Rule that would have devolved some powers of government to Ireland, and which Ulster Protestants usually opposedfearing for their religious rights... also not trusting politicians from the agrarian south and west to support the more industrial economy of Ulster."
https://en.wikipedia.org/wiki/Ulster
“In October negotiations opened in London between members of the British government and members of the Dáil, culminating in the signing of the Anglo-Irish Treaty on 6 December 1921...Treaty applied only to the 26 counties, and the government of the Free State had neither de facto nor de jure power in Northern Ireland.[13]
https://en.wikipedia.org/wiki/Irish_Free_State#Northern_Ireland_”opts_out"