I don't want to derail this thread but yes there is an Irish language. It is called Irish or Gaeilge (which is the Irish word for the language)
It is a living language which is spoken by a minority of Irish people as a first language today. It is also the first official language of Ireland, with English the second. Every Irish school child learns Irish throughout their Primary and Secondary school career though it is not taught very well.
Irish is a Gaelic language as is Scots Gaelic and Manx.
All three of which which fall into the Celtic language family with things like Welsh and Breton.
No one speaks Gaelic. People speak one of a number of Gaelic Languages which I listed above. Americans coming to Ireland and talking about Gaelic as if it is our language is a subject that will annoy most Irish people.
I don't understand why you would bring DNA or invasions into this. You also seem to have the order mixed up. The British came to Ireland much later than the Celts. The order would be Celts, Norse, British. Celts are probably closest to a native people that we know of.
The language is a living one, as is English and has changed throughout the centuries as does any language. The only historic fact of importance when it comes to the speaking of Irish is that the British did their best to stamp it out in the 1800s and did a very good job.
https://imgur.com/jjwsZO4
Trying to go back on topic I wonder if the Phillies will sign an Irish player in July...
