title_body: Would these features be naturalistic in this scenario?
upvoted_answer: Yes, the whole thing looks naturalistic.
  
  ad 1) When you assimilate the Old Norse words to a Goidelic phonetic system, lenitions and broad/slender distinctions come in very naturally. It will be more difficult when you insist on some Old Norse sounds sticking out of the Goidelic system; they can form exceptions to the lenition rules or evolve specific lenitions of their own.
  
  ad 2) Retaining the Old Norse suffixed article is definitely an option, and simplifying it to only one form (independent of gender, case, and number) looks most natuaral to me, but keeping the inflected forms is also covered by your artistic license as a conlang author. Or you may inflect the definite forms according to Goidelic inflectional rules.