"The Indo-Europeans were not largely Celtic, the largest numbers of Indo-Europeans were Aryans"
The Aryans (Indo-Europeans) had the caste system that the Celts (not the Slavs nor the Germanics tribes) used. It was the same social structure, same origins, similar language. This cannot be said of the Germanic tribes.
The Celts didn't ONLY settle in Brittany region, but the Irish and Brittany Celts you are reffering to were the only celts to do so.
You seem to forget the "Celtic" at this time period refers to almost 5 different groups. Brittany being one. (Made up Brits and Welsh) Irish being another (Irish and Scottish) Iberians (Spain) Then there were the Gauls and one whose names elude me. The first lived on the northern border of the Alps, the second lived on the western border of the Russian Steppes (Prussia) and made up a substantial portion of the "scythian" tribes. The last tribe is likely to have come (or at least have bloodlines into) the mongol hordes.
The Gauls also made the Galatians (from which we get the name of a book in the Bible.)