I always wondered if Irish Gaelic and Welsh Gaelic were the same. Now I know!
According to the Beginner’s Guide to Irish Gaelic Pronunciation, Irish, Scottish Gaelic and Manx are very similar, and “make up the Gaelic branch of the Celtic languages.” Welsh, Cornish and Breton make up the Brythonic branch, and it takes a linguist to be able to tell they’re related.
Pronunciation gets crazy, with extra vowels added to make the consonants sound like they need to (similar to our “soft C” and “hard C”). More on that another time.