Does someone know that expression :
ar captaen an eachtrai taoiseacht?

It was long ago that I read that phrase in a book so I might be false in writing properly that phrase.

I also wonder if Gaelic is close to the tongue in Brittany?

Thanks

I understand all the words there Alexis, but they make no sense the way they're presented, at least without some form of punctuation.Here's a stab in the dark:
Captaen = Captain.
eachtraí= events/adventures(I'd say it's "adventures" in this case)
Taoiseacht= Chieftainship.

It probably means "The captain of the action/adventure, should be chieftain."

Secondly, Breton in Brittany and irish in Ireland are both categorised as Celtic languages, but are not mutually understandable.But they have more in common with each other than with most other european languages.

Irish, Scots-Gaelic, and Manx are "Q-Celtic" languages, while Welsh and Breton are both "P-Celtic" langauges.

"Q-Celtic" because there is a 'q' or hard 'c' sound in Irish, etc. where there is a 'p' sound in Welsh and Breton. For example, ceann is the Irish word for head while in Breton and Welsh it is penn. (Penn gwin means white head and comes into English as what word?) Good that you mentioned that Irish and Breton are mutually incomprehensible. There is a folk belief to the contrary.

Irish is sometimes described as the most conservative of the Celtic languages and Breton as the least.

Where did you get the phrase, Alexis? Poetry? Prose? From an Irish text or was it in the middle of an English story? Good memory but there's still something missing. I wonder if 'ar' is actually ár here: ár gcaptaen = our captain. 'Eachtraí' can also be a noun meaning adventurer (or wanderer or exile); taoiseacht might be 'taoiseach' chief(tain). Our captain, the adventurer [and] chieftain? Still something amiss.

Truth in linguistics regulations require me to correct the spelling of what I wrote above: "penn" and "penn gwin".

In Welsh, it is "pen" and "pen gwyn".

In Breton, they are "penn" and "penn gwenn".

Now I can sleep at night.