England is able to retain their Angevin holdings in France but in exchange do not own Scotland (i have no idea how this changes things or how this would have happend realistically but i liked the idea)
basically the lore for its formation is that Saint Patrick's attempts at converting the Irish pagans fail, but not entirely. Saint Patrick, along with some of his close followers, establish a Christian Republic in County Down. I also imagine that many Christians living in Roman Britain would flee here during Rome's collapse, meaning it would have had a big enough population to defend itself from other Irish kingdoms. Irish does eventually become the predominant language but Latin is still in semi-regular use.