Branwen
Branwen, Daughter of Llŷr is a major character in the Second Branch of the Mabinogi, which is sometimes called the Mabinogi of Branwen after her. Branwen is a daughter of Llŷr and Penarddun. She is married to the King of Ireland, but the marriage does not bring peace.
Read more about Branwen: The Story of Branwen, War Against Ireland, Branwen's Grave