That was some impressive Irish expansion, with the best of warriors saved for last.
Oh yes, CK3 may be more balanced than CK2 but at a certain point, critical mass is achieved. That being said, we never had a truly massive war with a genuine rival since Ireland v England over Wales.
Starting a war with the Papacy to get a single province on Sardinia was questionable, but then again, I'm sure the Emperor was getting bored from the lack of challenge in his other wars on top of the usual frustration with unnecessary border gore...
A bit of 'lets settle this last bit of border gore' and a bit of 'I wonder what a proper big war between two proper big empires would be like?' As it turns out, rather fun and bloody. Makes me wonder what a multiplayer game with three or four big empires would be like...
What comes after 1400 would be interesting as you have just one major power with a long Atlantic-facing coastline, so there is unlikely to be much competition for the colonisation of the Americas, unless the massive Irish empire breaks apart.
I don't think the Empire will have many problems for a while. They have a lot of Europe to colonise as much of it is now empty and abandoned nature. France and Spain need to be repopulated withb Irish settlers, Germany and Denmark are pretty full of people already but the absolutly gigantic gians out east are going to sate the appetite of the population for another century at least.
The tricky question is whether this lack of competition would make the process go faster because less military force is required to protect the colonies, or slower because there is less of a rush to take territory before a competing power does. Sometimes regardless of economic or strategic usefulness.
Well...there's not much reason for us to look west. We have so many resources and trade power that we can deal with the Byzantine (who have a stranglehold on the silk road and far east land trade) on a generally even footing. We have absolutely tons of furs and raw resources to find out east in Scandinavia and eastern Europe, and that's not beginning to count the colonization rush to grab as much of Russia as possible.
So...there's no reason to settle North America (furs, wood and that's it, for the most part). Greenland probably will be found eventually by someone either looking for it for the Irish, or fleeing from their wrath.
As for the Caribbean...it'll either be me or the Papacy who find it, as we send down trade ships along the west coast of Africa trying to find passage to Asia. Very possibly them, because they must have a greater naval tradition and a bigger rivalry with the Byzantines. Now...the Pope finding the new world will be a big deal, because conversion! So the Caribbean at least will be found and settled (or at least looked at and maybe claimed) along the same lines we might expect. It's good for growing various tropical crops as well...
South America...no idea. I bet a holy crusade against the blood thirsty infidel may well be on the cards if the papacy finds them first. Or even second. Quite frankly, whilst the Irish should be the big colonizers because of their inherent advantage, it will probably be the Papacy instead, because they need a fuck ton of cash and goods to win wars against the Byzantine Romans.
That being said, they won't be able to hold onto them if the Irish or Romans come knocking. Smaller population and surrounded, so probably going to have some nasty wars in the Balkans or in Sicily at some point in the 16th century to decide who owns these areas once and for all.
And the reformation may or may not occur, hitting them where it really hurts. If Christendom sort of loses unity, and the Irish, the Byzantines (and Hungarians) and Papacy all have different faiths...well...chaos ensues. The 16th or 17th (more likely) century will be one of absolute bedlam.
We may actually see a world where there are multiple European empires...but no or few European colonies after the 16th century.
A EUIV continuation would be fun, though Ireland might well end up taking over most of the world by the 1800s, assuming the empire doesn't fall apart which isn't that unlikely of an outcome.
The Irish are going to have some huge problems at some point as a matter of identity. Unless they really wipe the slate clean, they are going to have some minorities remaining in their realm, and the frontiers are going to be fuller of them than usual. I would expect them to continue to push east, try to claim all of Russia and Siberia, and try to define the Ned and Irish parts of their empire cohesively. As it stands, this is much more a Hapsburg deal of an empire the just so happens to be ruled by one family, rather than a unified polity.
The exceptions being the Irish Isles, and Germany (at this point).
So yes, very interesting jumping on point for EUIV continuation...not sure if that happens or not though!