I am interested in Celtic languages and I am curious to find places where they are spoken in public. Is there any known places in Britanny where Breton can be heard casually in public?
That won't be easy.
It is now taught in schools, and used in bi-lingual signposting across Brittany, but it is not a language spoken in the workplace, and, perhaps even more significantly, it is not a language spoken in the countryside by farmers or at local markets.
In the centre of Brittany there are still bars in the small villages where some of the original Breton dialects are still spoken.
As a non-Breton speaking Breton guy, my best guess is bars around fishing ports. I'd try any port west of Lorient, but the Nothwestern part of Finistère is where the most breton is spoken "casually".