They will flee your realm, so don't revoke titles from excellent and irreplaceable council members. Church taxes are trickier because Catholic bishops will pay taxes to the Pope if they like him more than you.ДЄlternatively, you could revoke the city and temple holdings from vassal mayor and bishop. You should probably pick the ambition to improve stewardship or the Business focus. Improving your character's stewardship attribute increases your demesne income from all provinces, and also allows you to hold a larger demesne. Try to invite a good steward to collect maximum taxes in your capital (or richest province). Once you hold the Kingdom of Ireland, you will be able to use this on any county in Ireland regardless of whether you or your vassal holds the duchy. This kind of war can be declared to vassalize or claim any county in a duchy that you or one your vassals personally holds. Whether you have the Jade Dragon expansion or not, you will also occasionally be able to use the de jure claim on county casus belli during your expansion, which costs nothing to use and should almost always be your preferred option when it is available. The Jade Dragon expansion also gives you access to the border dispute casus belli, but it comes with a nasty opinion penalty with all your vassals and the Pope, and you should not normally need to resort to using this in order to form the Kingdom of Ireland. You also still have the option to use your chancellor to fabricate claims, as described above, which will cost you some gold and prestige, and is cheaper and much more flexible because it can target any county, but requires you to wait an unknown amount of time before it fires. These kinds of wars cost gold, piety, and/or prestige to declare. The former allows you to claim another county in a duchy in which you already have a county, and the latter allows you to vassalize a weak neighbour of lower rank.

If you do have the Jade Dragon expansion, your primary means of expansion in the early game will be the ducal conquest and force vassalization casus bellis. If a character is dissatisfied enough with their liege, this could lead to you getting a Chancellor with a Diplomacy ranking of 21 at the very start of the game, making the fabrication of claims pitifully easy. Alternatively, you could try to invite a character with high Diplomacy to your county from overseas. This goes for any game where you begin as a count. If you don't like waiting and twiddling your fingers, restart the game as much as necessary until you get a decent one (I recommend 16+). The stats of your courtiers is randomized at the beginning of every game, and the fabrication of claims will proceed far quicker with a good chancellor than a poor one. If you don't have the Jade Dragon expansion, your primary means of expansion in the early game will be fabricated claims, so you will want to have a chancellor with a high diplomacy score.

Ck2 how to get claims on kingdoms