One point among several:

Welsh and Scots Gaelic and Irish and Manx and Cornish and Breton (and some others, mostly extinct, including Gaulish) certainly are related, though you wouldn't guess it just from reading or listening to them.

All can be shown to descend from a common ancestor spoken before roughly 500 or 600 BC and known to linguists (but not to the speakers) as "Proto-Celtic." Proto-Celtic evolved into two later languages, known to us   as "Goidelic" and "Brittonic." These led to the development of the modern Celtic languages.

The Wikipedia article looks pretty reliable but definitely isn't for the faint-hearted:

https://en.wikipedia.org/wiki/Proto-Celtic_language