The Celts were a group of people originating in southern Germany/Alpine regions somewhere around 1000 BC. They spread across Europe either conquering or assimilating the local populations until they reached the edges of Europe. The Celts settled Gaul, Britain, Ireland, Spain and Portugal and tried to invade Italy and Greece but were driven back. I think they may have settles in the Balkans or Asia Minor (I have vague recollections of an area settled near Greece, but cannot be certain). The Celts of Ireland spoke an older dialect to those of Gaul/Britain and may be thought as the original Celts.
In my schema, the Druids encountered the Celts of Britain and Ireland and became their priestly class, spreading through Gaul until they reached the German Homeland. Their attempts to provoke the Gauls led to Julius Caesar invading Britain and destroying their stronghold on Anglesey.
After the Roman Invasion, the Celts of Britain became Romanised to a certain degree. Those in the South and East became more Romanised than those in the West and North. The Gauls became completely Romanised and the Celts of Spain and Portugal eventually were Romanised, although they play no part in the Age of Arthur. The Irish Celts were left alone by Rome, but must have been influenced to a certain extent by 4 centuries of trade/raiding. Certainly, St Patrick had some influence there.
The Romans tended to assimilate local cultures by equating the local gods with their own gods and thus degrading the local deities. This was similar to the druidic practice of making pantheon identifications and even allowed some druids to gain spells from the Roman pantheon. In Britain, this practice had the effect of diluting the Celtic Gods. Christianity had a large effect - when the Romans left Britain, it was largely Christian in the South and East, but pagan in the West and in the North.
In the century before the Roman Departure, many Britons crossed the channel and settled in an area of Gaul henceforth called Brittany.
Soon after the Romans left, the Irish started raiding and set up various kingdoms, some in Wales but the most important one in Caledonia, what is now Scotland. This kingdom was centred around the Hebrides and was peopled by the Scotti, an Irish tribe.
Thus, we have a patchwork of people with a similar culture spread across Britain, Ireland and northern France, all of whom were Celts and all of whom were different.
The Irish retained the Hero Rage ability which they had of old and which the Britons seemed to have lost. The Irish worshipped a different set of deities to the Britons, although some were common (Brigid, Bran). Most people only worshipped one deity, but some worshipped more than one, especially powerful warriors and chieftains.
Once again, in Ireland I have a quick list of the major Irish deities and the spells they grant, but I do not have them here. Annoying really.
By the time of Arthur, many of the Irish had been converted to Christianity, but the conversion was by no means total. There were three sorts of Christians, those who foreswore all pagan deities, those who paid lip service to Christianity (lay members) and still worshipped pagan deities and those who worshipped pagan gods and Christian Saints, having been taught how to do this by the Druids.
The Britons of the South and East were generally Christian but had been conquered by the Angles and Saxons by the time of Arthur. Many had fled to Brittany or to Wales, Cornwall or to the Men of the North. Those who remained had been assimilated by the Saxons as slaves or were too weak to resist. Often they co-existed with the Saxon invaders but did not raise arms against them.
The Britons of the West and North were usually pagans who worshipped a pantheon different to that of the Irish and Gauls. I will not detail them here as my notes are in Ireland, but they seem to be able to be modelled in RuneQuest terms. Sometimes, the Celtic Deities had been absorbed by a Roman equivalent, so some Britons would worship the Celtic God, others the Roman God and sometimes even a Christian Saint with similar powers.
As a broad generalisation, the Britons of the West and North were more warlike than those of the South and East, perhaps because they had fought the Romans for longer and had resisted them, perhaps because some of them had never actually been conquered, perhaps because the Romans had destroyed all the warlike Britons in the South and East (the Iceni for instance) and perhaps because all the warriors had joined the army and had been marched away to other lands.
The people of Brittany generally sent assistance to drive the Saxons from Briton, but did not go there themselves. They did this by sending mercenaries across the Channel.