My suspicion is that it is likely, but not inevitable, and that whether it happens or not is a complex question.
The baseline, 1920 arrangement was the Northern Ireland was carved out specifically for the Protestant Anglo-Irish (these are all the people of English descent who settled in Ireland during the hundreds of years of English suzerainty over the island) and Protestant Irish, and its borders were drawn to guarantee that Catholics could not have a majority in the Northern Ireland Parliament.
However the Catholic population had generally always grown faster than the Protestant population. By current times, the Catholic population is around 40%. The Protestant population has actually declined–although that is actually more because many of them have become non-religious. Additionally many of the demographic assumptions have changed.
In 1950 you can damn well bet that almost everyone in Northern Ireland was either a Protestant or a Catholic, and you can damn well bet that if you were a Catholic, you favored union with Ireland, and a Protestant favored remaining union with the United Kingdom. But in 2020+ things are different. Much of the Catholic influx has actually not been from the ROI, but other EU countries…the so called “A8” countries. Many are from Poland. This means that many Catholics settled in Northern Ireland are no longer cultural / ethnic Irish Catholics, and thus they don’t intrinsically have the same views on the union–some of them actually specifically moved to Northern Ireland because of various economic opportunities there that are seen as being tied to union with the United Kingdom.
Meanwhile, and thankfully, the religious divisions (while by no means gone–there are still neighborhood walls that are sadly necessary to prevent violence) that have long defined the conflict have been decreasing. For example there have in recent years actually been Irish Catholics in the Unionist (Unionist means “maintaining Union with the UK”) parties, and the SDLP (the more moderate of the two big Irish Nationalist parties) have had a number of Irish Protestant members. This means that it’s getting much more difficult to guess what way things will go based on religious demography.
All that being said, as mentioned I think eventual reunification is likely, but not guaranteed. One thing that actually kept a lot of Protestants from wanting to reunify with Ireland for example was a perception that the Republic of Ireland is “religiously extremist”, remember that until very recently abortion was banned in nearly all circumstances in the RoI as part of its constitution. That was recently changed.
As to the question of will the UK grant independence–absolutely, they have agreed to do so if it is the democratically expressed will of Northern Ireland. There is virtually no chance they would attempt to block Northern Ireland merging with RoI.