title_body: How to develop a detailed, realistic a posteriori conlang?
upvoted_answer: So, you are going to create an altlang (a naturalistic language living in an alternate history of the the world). First, define your starting point (easiest for the first scenario: Old Irish or proto-Goidelic is a suitable starting point for this one). Look at the real world descendants from that starting points (Middle Irish and Modern Irish, Manx, Scottish Gaelish) and what changes happened to them. You can create variation by retaining some archaic features, applying changes from different descendants, or applying changes in different temporal order to create a new artificial Goidelic language. You may also throw in some original changes not occurring in the known Goidelic languages. Keep a timeline of those changes—the changes will apply to loan words as well!
  
  Than decide on the influence of the other language: Are there only borrowings of words, shall it influence syntax (e.g., the order of noun and genitive or noun and adjective, shall the basic word order change?), are there new phonemes added to the phonology? 
  
  Decide, when the influence happens, does it happen only once, are there several waves of influence. Note that the other language changes over time, too. Your conlang may preserve archaic words from the donor language that have fallen out of use or have undergone sound shifts there.
  
  Put everything together and enjoy the result!