Branwen is the daughter of Llyr and Penarddun, sister of Bran and Manawydan, and half-sister of Nisien and Efnisien.
Matholwch of Ireland sued for her hand in marriage to create an alliance between Ireland and Wales.
Angry that no one had consulted him concerning the marriage Efnisien mutilated Matholwch's horses. Matholwch considered this a grave insult and commanded his men to return to their ships in order to return to Ireland.
Bran tried to make amends for the insult by replacing each of the mutilated horses, and by giving Matholwch a staff of silver as tall as he and a platter of gold the size of his face. Despite these gifts Matholwch was still angry. Bran then gifted him with the most prized possession of Wales: a magical cauldron that would restore the dead to life.
This seemed to appease Matholwch and Branwen wed him.
Matholwch returned to Ireland with Branwen. where, within a year she bore him a son, Gwern.
Even this did not bring any warmth into the marriage as resentment about the insult lingered and the Irish began to complain about their foreign queen.
She was soon banished to the kitchen, where she was a slave and boxed on the ears by the butcher daily.

To keep Branwen's situation from reaching Bran Matholwch banned all ships from sailing to Wales and imprisoned any incoming Welsh crews.
This lasted three years.
During this time Branwen trained a starling to speak and sent it to Wales, where it told Bran of her plight.
He immediately sailed to rescue her.

In Ireland, Matholwch was told of an unusual apparition - a forest had materialized in the sea! No one could explain to the terrified Matholwch what the sight meant, so he called for Branwen. She explained that it was Bran's navy with Bran himself wading through the water at the forefront.

An alarmed Matholwch came up with a plan to rid himself of Bran. He built for Bran the first house ever that was big enough to hold him. From every pillar of the house he hung a sack containing an Irish warrior. At a signal from Matholwch the warriors would emerge and slaughter Bran and his army during a great banquet. Bran was very impressed with the house but Efnisien was suspicious of the sacks. Matholwch told him that they were just sacks of flour but he wanted to inspect them. Upon close inspection he felt the shape of a mans head. Efnisien squeezed the head between his thumb and forefinger until they met in the middle. He did the same to each of the sacks in the house before finally declaring himself satisfied. Somewhat disgruntled at Efnisien's actions, Matholwch had to abandon his plot. The banquet went on as planned though. During the banquet, in which Matholwch agrees to abdicate his throne to Gwern, Efnisien asks if he may caress the child. Permission is granted and Gwern approaches Efnisien from across the banquet hall. When the child came within arms reach Efnisien grabbed him and threw him into the fire. Immediately the banquet turned into a bloody brawl of which the Irish soon had the upper hand because they had the magical cauldron that would restore the dead to life. Efnisien, repenting, sacrificed himself by hiding under a pile of Irish corpses and being thrown along with them into the cauldron. Since no living thing could be placed into the cauldron it shattered into 4 pieces killing Efnisien at the same time. After 3 days of bloody fighting Bran's army was victorious, but at a terrible cost. His huge army had been reduced to only 7 men - among them were his brother Manawydan and Pwyll's son Pryderi. A poisoned arrow in the foot fatally wounded Bran. On the Irish side only 5 pregnant women remained to repopulate the whole island. Realizing that the two kingdoms had been destroyed on her account, Branwen sailed to Aber Alaw and died of grief.