You can get this by playing through the tutorial and conquering all of Ireland but if you skipped the tutorial or already finished it without taking the whole kingdom start a new game in 867 on IRONMAN mode (Easier year cause the Norse men will be trying to take the British isles and leave you be for the most part) Chose a ruler in Ireland (Petty King Murchad of Munster is who I would use and is who you use in tutorial) and start your conquest. Once you take everything or vassalize everyone in Ireland you have to buy 1 more duchy title since you start with one and then you can buy the kingdom of Ireland title. Duchy title cost 250g and Kingdom titles cost 500g. Ducal Conquest in the Diplomacy skill tree will reduce title creation costs by 20% and Enables the Ducal Conquest Casus Belli. This can help if you dont have alot of money and help you take land without having to fabricate claims as much.