Amazing responce! So many challenges to try, and in return I give you my other!
Muhammad O'Fatimid
(If you don't have ruler designer, I apologize!)
As you may have guessed, you must redesign a count in Ireland as either a Bedouin, Levantine or Egyptian Muslim (you may pick shi'a or sunni) and form your own realm! You CANNOT convert, your characters see themselves on a divine mission from Allah to take your god given right to rule x county, and spread that right to the whole island of Ireland! And beyond! The challenge ends when you are King of Ireland, or if mods allow, Emperor of Ireland/Scotland/Britannia/British (Whatever you prefer). Your character is a nobody who's father bedded foreign noblewoman, on his deathbed he sent for you, and demanded his vassal lords to accept you. You are count/duke for now, but you'll find that the Irish don't take kindly to foreign heathens.
Restrictions:
-NO NO NO CONVERSION DAMNNIT! You may become a heretical muslim, or even switch braches, Allah knows why you'd make it more difficult for yourself.
-You cannot become irish cultured until you are three generations in, so your grand-son may manually become Irish. If you are given the option beforehand, you may change culture.
-You may marry Irish courtiers, there is no ethnic purity involved.
-You don't have to be a Fatimid, the title is a joke. If you have the customizer feel free to change your dynasty name to your first major title, or a meaningful event.
-You may join Jihads.
Good luck!