The Mabinogion is an assembly of Welsh Stories that were taken from Two Ancient books called the 'Red Book of Hergest' and the earlier  book 'The White Book of Rhydderch' . They tell the stories of people in ancient British Celtic times, some of the stories have been placed from approx 500 BC.

I found this document HERE These Manuscripts were Translated by Lady Charlotte Guest, some doubt has recently been expressed by several historians to the accuracy of her translations.