How do I into this game? Everyone always tries to claim my shit

Try to get claims and get land for yourself, vassals owning land is necessary but having a vassal owning more land than you is more often than not a really bad idea.
You can plot to revoke land if someone owns land in your capitals de jure duchy, so always take land in it.
And starting in ireland is the most difficult in the old gods start date, start either in 1066 or Charlemagne

The UI man, lots of going on.