I abandoned my only Irish game because the default names were too foreign for my taste. It removed my desire to spread Irishness all over Europe. Which is somewhat silly I guess but it just annoyed me.
I highly respect your opinion, no matter for that.
But the fact is that I never read one native Irish name before playing CK1, and loved to play them
because of their names.
And I sincerely believe that the more difficult in a game is a 'false' immersion: I'm thinking here of Normans having a few English names, of Egyptian Arabic women having names of Old Egypt goddesses, and so on. I could never play Egypt because I didn't wanted to see more Isis and Sekhmet on Earth!
But what's the more chocking of all: English names for the popes! What a shame! Yet that has been said, so I won't stress this point. I just suggest to read a more generalized conclusion to Olaus Petrus:
Using English names instead of foreign ones is cultural imperialism.
(Thanks for lending me you sentence, Olaus!)
This isn't aimed against you, but English speakers sometimes lack cultural sensitivities when it comes to names. Personally I have answered few times to questions like: "Is that even a name?"
I believe this is not the best reason. I'm allways surprized to see so much and so various names and surnames in USA, where they're used, pronounced and writen down without real problem.
So, I believe the real problem is in fact that they're not--generally speaking--used to practice another language than English, so these names having another rules for spelling and pronunciation are quite a Hell for them.
And if this would be considered an injure, that's not the case at all. Being French, I perfectly know this state of mind, even if French language is not as extended as English!
Of course, the great thing about Irish orthography is that once you know the rules, it's almost completely regular. That goes for Modern and earlier varieties.
Okay, so there's just to learn that each 7 letters equal one sound. Quite easy, in fact!

This time I understand English people!
End of joke: nobody has a better list for Celtic names?