Did the Celts exist? Is "Celtic" more than a cultural label? If they did exist, who were they, when did they thrive and when did they lose their cultural identity? The very word "Celt" is divisive. To some it is symbolic of independence from England. To others the whole idea of Celtic unity is a modern myth. To others still, the Celts are a long-vanished people who once burned Rome and attacked Delphi, whose descendants are scattered throughout Europe but have been subsumed within other tribes and cultures, losing their Celtic identity everywhere except in the corners of Brittany and the British Isles.

Simon Young is familiar with these views. Indeed, he begins his book with an account of a dinner party in Spain that ended in acrimony simply because of a passing reference to the Celts. That unpropitious event is a good starting point, for it allows him to explain his own answer to the "Celtic question". In short, he says, the Celts did exist, and in this book he shows in three ways how they affected the development of western civilisation.

The first part of the book is given over to the Celtic domination of northern Europe before the Roman empire. Rome itself only once succumbed to an attacking force  when it was burned by the head-hunting Celts in the early fourth century BC. The same tribe also defeated the Romans in pitched battle  quite an achievement considering their tactics involved stripping naked and attempting to scare their enemies into flight by a sudden loud and vicious charge, wielding long swords. Most importantly, they wiped out a number of Rome's enemies, leaving the field relatively clear for the Italian state to dominate in later years. Macedonia, for example, the only Greek political entity that looked likely to develop as a Mediterranean empire, saw its king killed by Celtic attackers in 280 BC.

The second part of Young's "Celtic revolution" concerns the explosion of Celtic missionaries from Ireland in the sixth and early seventh centuries AD. Centuries after the fighting heroes had finally been swept from the battlefields of Europe, their equally fervent and heroic kinsmen in Ireland chose to practise extreme worship through exile, either by flinging themselves out to sea into leather-covered boats or by covering Europe on foot. Those who believe that Augustine's arrival in 597 marked the start of Christianity in England may be surprised to hear that Northumbria and much of the north was converted by Irish monks from Iona, acting independently of the Roman mission. True, the Celtic rites of Christian worship were eradicated in the seventh century; but by then the Celts had done their work. Just as Rome's rise to pre-eminence owed much to the Celts along the way, so too the Roman church's conversion of England owed much to Celtic missionaries.

The third part is the most subtle. Think of Arthur and Guinevere, or Tristan and Isolde. The early medieval "romance" poems might have been entirely fictitious, and they were certainly embellished by non-Celts along the way (normally being written in French), but in the 12th century there was a deep quarrying of the ancient literatures of Ireland and Wales. The Arthurian stories sprang with joy into a Christendom heavy with incense and piety. Men and women yearned to hear stories of magic, heroism, lust and passion  not just the pious love of the Christian world  and they found it in the long-forgotten Celtic myths and legends. One might say that, having converted large swathes of Europe to Christianity, the Celts also provided the antidote: Arthurian courtly love.

These three themes do not describe a single "Celtic revolution" but rather three distinct cultural confrontations. The title is therefore a little misleading. However, the book has a great deal to recommend it. Young wears his considerable learning lightly  not many scholars can describe 2,000 years of European history with authority  and his style is light, witty and enjoyable. Certain descriptive metaphors stick in the mind, such as the late Roman empire being like an ostrich egg in a vice that the barbarian peoples on the periphery were turning millimetre by millimetre. Or the Irish saints who launched themselves upon the sea in coracles being the "dandelion seeds of the Lord".

But best of all is his way of making the scholarship work for him rather than being a slave to academic conventions and inflicting them on his readers. He has taken current thinking and used it with a specific purpose: to suggest a different way of thinking about the Celts. Rather than regarding them as a strange horde of shrieking barbarians always on the fringe of events, who were eradicated from the cultural mainstream, we might understand their history as central to European history and culture.

Did the Celts exist? Of course they did. They are all around us.

Ian Mortimer's books include The Time Traveller's Guide to Medieval England (Vintage).