So this would suggest that the Goidelic languages (Irish Gaelic, Scottish Gaelic, Manx) are derived from the Brittonic languages (Welsh, Breton, Cornish). [1]
I wonder how this affects the different proto-Celtic to modern Celtic languages hypothesis. [1]
This would suggest the following language tree: Breton -> (Cornish / Welsh) -> Irish Gaelic -> (Manx / Scottish Gaelic).
Linguistic lineages are not the same as genetic lineages. Populations can switch languages, or die out completely while their language lives on. And in this case, genetic studies show no clear genetic relationship between the Celtic-speaking regions - the Cornish are mostly English, for instance.
There were no Brythonic languages 15,000 years ago.
What are you talking about?
This migration predates the arrival of Celtic speakers by many thousands of years.