The Eóganachta
In southern Ireland there are 3 counties of Eóganachta dynasty rulers. I never played Ireland as a noob land, and always took it very seriously and often expanded, bringing the Irish to the rest of the world. When I decided to play in Ireland I took a good look at the different dynasties and found a ton of information about them. I fell in love.
Before reading too much I made my choice. Eóganacht-Locha Léin in the western county of their kingdom of Mumu. Ireland has a few different dynasties that have different branches like the Eóganacht Chaisil who rule the same county but in the 867 start, or the Eóganacht Raithlind or Eóganacht Glendanmach. I personally choose them and just stuck with them. They became my Irish home team even though I'm not Irish at all myself. There are others as well like the Uí Briúin Ai and Uí Briúin Bréifne, also another interesting one the Ua Niall Noigiallach which is Niall of the Nine Hostages (cool name?)
I learned a ton of Irish history due to delving deeper into all the different groups. At one point they battled these creatures called the Formorians! Similar type of good stuff like the old Norse legends.They're also set up NICELY to go fight in Hispania as some believed they had originated from Galacia. Move over Asturias, we got this!
Ireland is an awesome magical place, don't overlook it because it's sometimes considered noob-island. Don't bother with England or Scotland, just form Ireland and then branch out from there and do something else. The Empire of Britannia will just bog you down, save it for another play through. I took things slow, first battling the other Eóganachta to become the king of Mumu first. The King of Tara was their unofficial high king, so I made sure to not snatch that title before I felt I was worthy to hold it. Eventually conquering the rest of Ireland but maintaining Eóganachta dynasties and letting them rule land. I tried to keep as many of the old Irish houses alive as I could. You know how it is though, some houses are just total bastards and lose their heads.
Widukind -
https://en.wikipedia.org/wiki/Widukind
You cannot play as him from the very start. He only shows up if Charlemagne takes Saxony. He will have a special rising up event with a bunch of troops that usually take back Saxony. A unique part of this is that Charlemage can choose to destroy an important holy tree which I'm pretty sure he always does. This add's to the glory of Widukinds rise. As the player though I suppose you could choose to not destroy it.... SO, if you jump in at that point as him you have the choice of either trying to continue on as a pagan or go Christian and be on the forefront of battle with pagans to your north and east. Historically Widukind lost the fight and then ended up turning Christian, but what you do with him is up to you.
You'll have to use the console to go into Observer mode to let time pass until the event happens. This will depend on Charlemagne a lot. You could also simply start as Charlemagne and insure it happens and then just switch over to Widukind.
edit: Or take the fight right BACK to Charlemagne and see if you can beat the Franks.