title_body: How to develop a detailed, realistic a posteriori conlang? Background
  
  To begin, I’ve been conlanging ‘properly’ for around two to two and a half years now and have been into it for far, far longer. However, I have yet to create a conlang that is truly in-depth, detailed, and realistic. I soon want to begin that process. I am confident in my knowledge of linguistics and have at least a basic knowledge of most areas and a more in-depth knowledge of others (none the less, I am still an amateur of linguistics, of course).
  
  The current situation
  
  At the moment, I am in the very early stages of planning – I’ve yet to decide on the final concept which I am going to develop. My ideas currently on the shortlist are:
  
  
  A Celtic language spoken on a (fictional) island nation off the north-western coast of Ireland with North-Germanic influence due to interactions and mixing with the Norse people throughout history.
  A Turkic language spoken in a (fictional) nation in the Southern Caucasus with influence from Georgen, especially on the phonology and vocabulary side.
  A Uralic language spoken in Eastern Finland and the Karelia/Murmansk regions with Slavic influence.
  
  
  Of these ideas, I am currently most drawn towards the first bullet-point, however, choosing the scenario of the conlang is not the crux of the issue. That is the issue of creating an in-depth and realistic, believable a posteriori conlang. I want to create a language that has detailed and believable phonology and grammar. 
  
  My questions to you
  
  
  What is your advice on creating a good a posteriori conlang in general?
  How do I effectively develop a conlang with as much detail and depth as possible?
  
  
  Thank you very much for any help or guidance.
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!