I almost always provide a "Jewish Homeland" within my realms (if I can) but I always make sure they're still of my religion so as to save them from other vassals as well as prevent bad opinion modifiers. It's pretty easy to get too, you just park your chaplain on Proselytize in your capital county and then it takes care of itself when you get Jewish courtiers. The Matrimarry the Jewish courtier to a daughter, wait for him to be converted and then land him. He'll raise the kids with guardians of his own culture and (now) your religion and boom, you've got a Jewish province. Bonus points for it being Venice and you've got Jewish Merchants of Venice.Well, you know, you have to have Jews. You won't succeed in Crusader Kings if you don't have any Jews.
But yeah, that does suggest it's a question of culture groups; I guess Irish is close enough to Welsh and Breton for government work.
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