The Celts were a tribe of Central Europe who had a class of traders (which culturally colonised all of Britain, among other places), a class of warriors (which defeated and occupied Rome, among other places), and a class of Druids (in which school Pythagorus, among others, probably studied)
They never had a centralised empire and they never wrote their knowledge down. That's why they were so easy for the Romans to obliterate and why all their knowledge has been lost.
They were certainly not rubbish. They were possibly the most advanced culture yet seen on Earth.
The Irish, Cornish, Welsh, Manx, etc. are not racially Celtic but culturally Celtic, and as a rule are rightly proud of the fact.

John