Originally Posted by
Closet_Skeleton
No need to revoke it really. When your king of Ireland you'll want somebody to give the duchy of Leinster to. Its better to have some random guy as a vassal then a brother with a claim on your title.
You can own every county in Ireland in your personal demesne. But that requires you to get your son a stewardship education when really you want a diplomatic one. Its just too much hassle, you won't need the money after you've got 5 fully upgraded counties and if you're diplomacy is high enough you can get away with taxing feudal vassals anyway.