No, I can't remember there ever being any Irish-themed shires in Hyperion. I haven't actually ever read about Irish culture in the feudal era, but it sounds like a cool idea.
The way it works is that already functioning and active clans choose to become vassals. When a clan gets going and learns what they're all about, what they can offer as a vassal and what they need in return from their liege lord, a fealty can be discussed.
The Kingdom is led by His Majesty King Manus, who through "The Crown" constitutes the supreme executive and judiciary government. As for legislation, there is the House of Lords consisting of Peers (tenant-in-chief Barons and higher), and the House of Commons consisting of knights. Laws are passed when voted through by parliament and signed by the king. Shires are free to create their own local laws in addition to this.
The smallest unit that can be made a vassal is a Serjeanty, led by a landed Serjeant. Since they are so small, Serjeanties are usually made vassals of existing shires of the kingdom.
A Thane is traditionally a variant of the knight role for leaders of clans who may not be very feudal in their own internal clan structure.
Baronets are traditionally a civilian variant of the knight. I don't think we've ever had any landed baronets since all our vassals were usually warriors. A baronet can technically still be a military officer, but it's rare since historically only knighthood was associated with chivalry and heavy mounted combat.
It takes a lot of time and energy to build a clan. Normally you want a couple of loyal friends to found it together with, so you have some trusted right-hand men. Having a very good idea on paper, and finding some interested co-founders on a forums can work too.
Those with a social talent for leading and rallying people around an idea or cause can become knights very quickly, since that's very much what being a knight is all about.
edit: Ops, I forgot to actually link the Carta Solis.
http://www.lumensolis.com/showthread.ph ... arta-SolisThe section in which it is posted, you also have a lot of Oaths of Fealty sworn by various clans over the years.