Originally posted by Lhunithiliel
Oh, thank you! 
What I am so very much interested is to learn about those Celtic tales that have found some reflection in Tolkien's works and more than that - the historical backround of the Clets and their culture.
What I was thinking reading the posts in the present thread was that the Celts (if I'm not mistaken) were considered the natives on the British island. But they themselves had once come from Northern Europe. So, their culture must have derived from the culture of the Northern peoples.
So, you see, I'm looking for the whole historical "line", as history is the ground for the culture of a nation. Right?
So, it would be extremely interesting to trace the "roots" of a tale by Tolkien - does it have them deep back in the history of the island and its people or do they lie in the ancient culture of the Scandinavian peoples whose culture is said to have strongly influenced Tolkien.
Paralels! This is exactly what the thread is about! Right? 
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Browsing through my favourite encyclopedia - the Tolkien Wiki, here is what I found.
OMG!!!!!
If I have to read ALL that stuff!!!!!!!!!!!!!!
Now "The Early History of Britain" wouldn't that be a nice topic for one of the lectures of your guild?
I'm probably not the best suited member to give a sketch of the British history - since I'm neither British nor am I well-read in British history - , but since none else did it here so far, I'll give it a try:
Britain was part of the European continent until some 7000 or 8000 years ago.
'In the days of the Great
Kings when a man could still walk dryshod from Rome to York...'
HoMeXII
Megalithic "culture" probably came from Spain and/or France, then (~800BC) came the Celts who had spread all over Europe from about 2000BC. Next were the Romans, the Celts were forced westwards (Wales and Ireland), and northwards. Scots (from Ireland) came to the North where they and the Picts, who also had resisted the Romans, remained independent. When the Romans "officially left" again, a troubled period began, Saxons, Angles, Jutes and Danes were the new invaders from the European North. When those turmoils eventually had settled some (~1000 AD) the Normans (from north-west France) invaded what Tolkien now saw as "England".
Tolkien drew from the mythologies of all those - Greek/Roman, Celtic and Germanic (he disliked the term Nordic) - groups to create his "mythology", but he dedicated it to "England", the land of the "Anglo-Saxons" as he probably saw it and I think his own Saxon roots played a certain role in this...
Regarding the Celtic myths that are to some degree woven into Tolkien's tales, there's this book from Lesley Ellen Jones who elaborates quite ponderously on that issue...