WE’VE all heard of the Celts. But how many of us really know who they were?

Ask, and you’ll probably get different answers. They were the ancient Britons who lived in these lands before even the Roman legions came, some might say; the red-haired ancestors of the modern-day Welsh and Irish, others.

There is Celtic music, usually of the Irish folk variety; Celtic myths and legends; and Celtic languages, Welsh and Gaelic.

But who were these people?

Simon Young’s marvellous new book sets out to tell us: and the tale he tells is a riveting one, spanning more than 2,000 years and an entire continent.

About 2,400 years ago, the Celts were a conglomeration of Iron Age northern European tribes who delighted in warfare. They first appear in the historical annals when a Celtic tribe known as the Senones invaded northern Italy in 390 BC.

Warlike and barbarous, they sported long iron swords and wore bizarre Mohican haircuts: and they tore a swathe through northern Italy before clashing with Rome itself. They defeated the Roman army – one of the most admired and disciplined of the day – then sacked the city.

It left a lasting scar on the Roman psyche, but in many ways it paved the way for Roman domination of Europe, Simon argues.

Rome quickly recovered and, by destroying other Italian cities who were Rome’s rivals, the Senones left Rome master of Italy. And, before long, master of Europe and most of the known world.

The Celts paid for their impudence in sacking Rome.

The Celtic tribes who had once dominated much of central and northern Europe were either assimilated or pushed back by the Roman advance. By the time of Julius Caesar, the independent Celts had been driven back to Britain. The Romans then invaded, and subdued them there, too.

By the time the Romans left, four centuries later, the British Celts had become thoroughly Romanised. Then the Anglo-Saxons came, pushing the remaining Celtic Romano-Britons to the fringes: Wales, Cornwall and Brittany. There, they continued telling their stories: including tales of a great war leader who would one day restore their lands to them.

But it was in Ireland, never conquered, that Celtic culture best flourished.

The Irish Celts adopted Christianity – and sent out a wave of missionaries to spread the word of God to a continent plunged into the Dark Ages by the fall of Rome.

Dr Young is a dab hand at this kind of popular history – he wrote the marvellous AD 500 a few years ago, a nightmarish vision of a Britain abandoned and destroyed once the Romans left.

In this book, he has some fascinating tales to tell: of the crazy Irish missionaries who took to the sea in nothing but skin boats and put their fate in the hands of God; of the bombastic, war-like Iron Age Celts with their songs and myths; and of how many of the great Celtic myths – including that of the war leader who would one day return to lead his people to victory – were adopted and taken over by Christianity, their original, shadowy beginnings utterly changed in the process. King Arthur and his Round Table were born.

A fascinating book, that brings to vivid life the people whose memories, myths and legends still haunt these islands.