That's a lot of land! Though how developed it is is another question.
As covered by this AAR history recounting, a page back or so, the vast majority of that land is empty wilderness or depopulated villages, because Ireland conquered it under the Almighty Overlord who put a great deal of everything and everyone to the torch, looted everything not nailed down, and then the black death hit.
Iberia, and everything east of Germany was esepcially hit bad by this. The slavic peoples might not even exist anymore, except in the byzantine empire. And as iberia was almost entirely Muslim ruled, the devout Irish cleansed it. With blood and fire.
So the empire is huge, but it's also empty. Aside from the home islands, and certain regions of Germany that have been under the empire for a longer, and also Italy because the pope was watching...
Byzantium is richer and much higher populated than Ireland is. Although the Ned monarchy has more loot hoarded up for their own means, since they just spent two centuries looting a continent and every kingdom upon it.
Basically the Romans and Papacy have stable and prosperous realms, whilst Ireland have a very strong monarchy and army, but rule over an empire of Ash. Rebuilding an economy and population that large may well be beyond them.