As the duke of Irombro, you can change the allegiance of your duchy to a different realm. At this time, Irombro is a part of the realm of Riombara.
You can switch to another realm under two circumstances: First, when at least one region of that realm is a neighbour of your duchy; and second, when you are in a region of the realm. This means you can switch to any realm you desire simply by travelling there and pledging your allegiance.
You and all your knights will automatically join the new realm.
Emphasis mine. That's how you do it. The same applies for regions instead of Duchies (You have to be in the Duchy of your choice).
