Assuming even a subpar monarch (2 ADM/month), it would just take 4 years' worth of administrative effort to convince all the local population that you are a rightful suzerain to the area, and should it ever get retaken/conquered by someone else, those residents will think YOU are one of those rightful to own a fifth of Ireland for a 50 years because you have worked on integrating it for four years prior.
Gosh, some historical accuracy right here. You should look up the history of, for example, Ottoman-held Albania. There are countless other examples of many places unable to be fully brought into the fold of their conquerors. You should be glad that conquest is actually not as hard as it was historically and requires just some years' effort. Yes, France, Spain, Austria are so stronk. Have they ever managed to cheesily annex half of Europe by 1600 historically? No. Not even close. This is a fantastic change because it made conquest actually worth a hassle without making it too tedious to deal with in the long run.
Was conquest too "cheap" before? Most likely.
Is this system better? Absolutely not, it just makes it impossible to spread at the rate it was possible before.
After a while, I realised my issue is with the whole bloody magic points system than anything else. It's no more realistic for a province to cost X or Y magic points I randomly determine from my ruler. If you want more historical accuracy, I'd rather merge some EU3 mechanics into the game.
Say you conquer a province, you start coring it automatically. For each development of that province, it takes you some months to automatically make it a core (say... 6-12 months per development?). The religious tolerance / accepted culture could add bonuses to the coring speed. Spend admin points (each month, ongoing drain like annexation of vassals) to speed the process up, while increasing revolt risk. Defeat the separatist rebels that rose up from that province? Perhaps a small boost to the coring process.
Create a new envoy type - magistrate, to help with that (can only speed up the coring of a limited number of provinces at a time).
It would be (in my opinion) more accurate from the history perspective and it would still hamper rapid expansion. Sitting on overextension (which I would also modify from static to exponential growth along with a small impact from the country size) for a while would make you deal with rebels.