It looks like you’ve decided against the conversion, but as a veteran megacampaigner I’ll give a couple of points of advice on the conversion:
EU4 is a very ‘blobby’ game. Big and rich countries tend to get bigger and aren’t threatened with the possibility of internal collapse in anything like the way they are in CK. As a result, your country is probably going to be too large to be viable as a game that’s going to be in any way interesting.
That said, there are a few things you could possibly do to spice things up. Reduce the development of your provinces to a very low level (which sounds like it would match your story anyway) and have a patchwork of non-accepted cultures. You could also make religions different. This will add instability to your empire and make it harder to hold together. You could also mod the game to start with very low legitimacy, stab etc. I’d also scatter a bunch of cores for revolter tags across the empire to make a break up possible.
As mentioned, EU4 has its limitations as a game when you are a massive empire with no realistic competitors in your neighbourhood. But there is an exception! China has unique mechanics with the Mandate of Heaven etc that are a good model for the sort of massive and dominant power that you are. I have never done this myself, but there may be a way to mod the game so that it identifies your Irish Empire as China and gives you those mechanics. You’d have to look into how to do that though.
Also, I’d always recommend using the converter in the first instance (which is excellent) and then make tweaks after that rather than a by-hand conversion - which I’ve done before for some conversions as well.