I assume you meant something other than Leinster as Leinster is in my direct control. My heir was the earl of Dublin so when he became the ruler he took over Leinster and Dublin as his Demesne. Then I revoked the title of my vassal from Ossory and invaded it. He was imprisoned as you said. I had to revoke his title again and Ossory became my Demesne as well. Now, I still have Kildare. My vassal from there somehow loves me. I don't want to invoke his title to get it. Maybe I should wait for him to die and try to create unrest with the new vassal without revoking the title.
Had an other question but forgot...