I would recommend: County of Leinster.
Remote location, not a vassal and not under a crown law. In addition, when your current ruler dies you will continue as count of Dublin with 2 provinces - a great start against the other Irish counts.
Will teach you some useful tactics to expand via war or diplomacy.
Once you have created the kingdom of Ireland you will learn what it means to be king. After that you can expand into Scotland, Wales and England.