Few weeks ago, I bought a book about Celtic mythology, and I found many things that were directly borrowed by Myth. Now, I know that this isn't new stunning discovery, but I found several interesting entries, which I display here:
Cu Roi (means "hound of a Roi") was a Munster king. He transformed himself into dreadful giant in order to choose champion of Ireland. Only Cuchlainn (Irish version of Achilles) had enough courage to oppose him. They both fell in love with beutiful woman Blathnat, but she expressed her love for Cuchlainn; Cu Roi took her to his castle by force. Later, Cuchlainn laid siege to the stronghold, and Cu Roi was betreyed by Blathnat.
Maeldun or Mael Duin was great Irish voyager. Saga about his voyage to avenge his father's death is mixture of Christian and pre - Christian ideas.
Rhiannon was a daughter of Hereydd, and a wife of Pwyll, a chieftan of Dyfed. After birth of a son, she was unjustly accused og eating the baby. She endured her lot without complaint, which makes her singular figure in Welsh/Celtic mythology.
Of course, these aren't a complete references from the book. I just tried to shorten and say them in my own words.