The Mabinogion is an assembly of Welsh Storys that were taken from Two Ancient books called the 'Red Book of Hergest' and the earlier 'The White Book of Rhydderch' . They tell the storys of people in 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.