To form the Celtic-Union you need the following:

• Was never end game tag*
  
• Not tag: HLR, PAP, GBR
  
• Admin Tech 10
  
• Own all of Ireland and Scotland
  
• Culture Scottish, Breton, Welsh or in the Celtic group

*Celtic Union is not an end game tag.

AI will form the Celtic-Union.

There are events for Scotland and Irish Nations to make it a tiny bit easier to beat England (e.g. one stack of rebels spawning in Wales, +5% Moral or getting unique generals).