I expect that even Wikipedia can be a starting point for research...there are usually some references cited at the ends of Wikipedia articles.

I remembered this:

http://www7.nationalgeographic.com/n...re3/index.html

...which may give you some kind of starting points. The map may be of use and the "learn more" page cites some reference works that might be helpful. This may be a deeper subject than it might first appear. I hope that this isn't just a "five page paper" topic because I can easily see that a "thirty page paper" would be just an overview of the Celtic roots of Brittany.

Best

AA