The tutorials are...not great. Apparently par for the course with Paradox games. I played through the demo a few times before the game arrived, so had some idea of what I needed to do to get started, but even now, with a world-spanning Empire, I'm still fuzzy on some things (did just manage to become Grandmaster of the Templars, though. Wee, free Holy Order!)
I suggest sending out your Councilor to fabricate claims on a region that's part of the same duchy. Then you can take it, usurp the Duke title (if you now control 50% of it, which in Ireland, you should) and then get de jure claims on the rest of the duchy.