I play as the King of Ireland (and Castille), and I've betrothed my male heir to the heir of a duchy and some stray counties in England. As I understand it, my daughter-in-law will rule as a duchess on her own, but their heir will rule both my kingdoms and the English lands (as they are of my dynasty).
However, the English lands are of course subjects of England. So what happens then? Will the duchy and counties be separated from England and become part of Ireland? Or will I become a vassal of England – and if so, to what extent?