Are you sure, that, the Spanish only let Castillians to the New Worlds? In our timeline, there were many Basques in the New World.
I won't be making overly strong claims about it, but it's my impression that the Germans, Basques, Greeks (Lebanese, Catalans, Polish, Hungarians, Italians etc.) are overwhelmingly, like 90%+, 19th/20th cc. emigrants.
There were however Irish settlers who came in the 18th c.