Gaelic and Hieroglyphics

Gaelic might come directly from the 1550-1320 b.c.e kingdom of Egypt inner royal language.

In most ancient societies; their were several layers of languages.

Starting with

Inner royal; the language the family used, for private conversations

Court

The extended royal family; including families which used to be enemies, or conquered foes(generations back enemies)

Upper class including business people

Most of the rest of the population spoke some version of common. But that common changed per generation per the youths behavior patter of creating new words to describe their perspective. Reference the last 150 years in America; Dandy, Gay, Brill, Excellent, Awesome, Cool, bees knees, etc.