Thank you both, that advice really helped (although I think I married off my heir matrilineally to a kingdom in Wales by accident and now my King is Welsh and my Irishmen hate me .-.).
I decided to start over, and did much better the second time around. I've been winning most of my battles by using your advice, and have also youtube'd some CK2 Let's Plays that taught me some important things, namely, why you don't want to have half your vassals pissed off at you.
I managed to declare the Kingdom of Ireland, but my succession is a mess. I'm at risk of losing a duchy at a minimum whenever my king dies, and I only get around that by carefully switching around succession laws. I'm guessing I should be gunning for high Crown Authority so as to force all of my land and titles to go to only one heir? It's a good thing that my kings have all been unusually long-lived which has allowed me to get enough time to steer the ship around and keep my titles united.
I then decided to restart again, but this time to play as the Kingdom of Leon. What followed during the reign of my first King was basically me taking all of Galicia, Castille, and Navarre, now regulated to Zaragosa, as well as managing to get all of Galicia, as well as Asturias and some other provinces directly held by me or my heir rather than vassals.
It was quite a glorious curbstomping, but I'm having issues keeping my vassals happy. They always seem to come to hate me no matter what I do. Is it the fact that I'm holding several dukes in prison for plotting treason? Or that I'm basically setting up my heir to directly own a third of non-Muslim Iberia?