Welcome.
Here are my personal three recommendations:
* Do not trust the difficulty in the character selection screen. Instead pick some Christian Duke as your first character. Ireland is for a good reason called the tutorial Island but there are other places which also are good for learning the system of the game while also having good time.
* Keep track of your vassal opinions and why they are angry at you.
* Tutor your heir by a guardian of your own culture and religion, otherwise the child might change culture which could end badly.