Continental Celts are the Celtic-speaking people of mainland Europe and Insular Celts are the Celtic-speaking peoples of the British and Irish islands and their descendants. The Celts of Brittany derive their language from migrating insular Celts, mainly from Wales and Cornwall, and so are grouped accordingly.
Being Celtic basically means being from a population with predominately male Yamnaya ancestry.
I am no fan of linguistic and cultural theories.
I like genetics. DNA doesn’t lie.
Male Yamnaya tribes were the Proto-Celts invading Europe for centuries and finally finding its highest y-DNA remnants among today’s Scots and Irish. The Basques are an exception. They don’t speak their language. They kept the Proto-Celtic language going.
When the Basques were invaded by the Proto Celts, most of their males were wiped out. Adult males. But boys taken into slavery didn’t tend to survive either. The women were left alive and wound up having the conquerors’ children.
However, it seems the conquerors didn’t spend a lot of time raising their children. The women continued raising them the Basque way meaning that they passed the language one.
However:
Most Basques don’t know their origins. It stopped when oral translation ended with the death of adult males. Fathers tended to be the ones passing on history.
From the beginning of our research related to people who test Rh(D) negative, Celtic ancestry has been of great interest to us.