These languages, sadly largely killed off in the name of I don't know what, are as the final Tolkien quote in the comment says: the senior languages of Britain.

Fun fact; Cumbria is named in the same way as Cymru, as Welsh was once spoken there too.

I've never seen a video where Welsh/Breton/Cornish speakers have tried to have conversation, but have seen a few Irish/[Scottish] Gaelic conversations.

And at one point way back they all would have been intelligible. Shame it didn't follow the Scandinavian evolution, or they may have had an easier way back from having a "prestige language" take over.

It's interesting that unmodified "Gaelic" apparently refers to Scots Gaelic in Wales. If you asked me about "Gaelic", I'd assume you meant Irish Gaelic.