The whole saga of the "celts", it is still believed, as evidenced by another current thread!
They never inhabited Britain, they only ruled it, they did not displace the ancient Brits, because the ancient Brits are still here, they were not displaced by Anglo Saxons because no one was.
The so called " Anglosaxons" were just a rag tag mercenary force of assorted ex auxiliaries retired from the Roman army employed by ancient British tribal leaders to support their infighting.
The whole story of the nobel "Celts", as a tribe which inhabited the UK and were driven into the Western extremes was invented by the Tudors who had no substantial claim to the English throne, but came from Wales, so invented this whole story about the true inheritors of Britain having been driven into Wales, and now reemerging from Wales
Their propaganda machine was so good people still believe it today. The tribe we call "celts" only ever inhabited the Western fringe of the UK, and dominated the whole island from there, there technology, political and military structures etc were so much more advanced than hours, they set up and upheld rulers to rule the whole of Britain.
So places like Ireland were English long before they were "Celtic" and in fact nobles sent their sons to Ireland to learn proper English there, before they were galic speaking!
And as for were this tribe even "Celtic" - no they weren't, nobody seriously believes that these days. That was a Victorian invention, much like Scottish clan tartan....
yet their are peole who's identity is so closely bound up with these old fairy stories that they can't cope with the truth.
Says a lot about the stupidity and pointlessness of choosing and clinging to an "ethnic identity"