During the Fenian Cycle in Irish mythology, Muirenn was the daughter of Tadg. Tadg was a Danaan druid and sorcerer, who was one of the advisors to the high reigning king at the time, Conn Cétchathach. Conn had created a military order that consisted of the elite warriors of the land called Fianna Éireann. This order comprised many clans, but the two most powerful clans were the Baiscne and the Morna. Out of each clan there was a chieftain, but only one warrior could gain enough support to be captain over all of the Fianna. During Conn’s reign this warrior was Cumhaill.
Since Muirenn often attended the king’s court, she and Cumhaill fell in love through these meetings. Although her father did not want her to marry a mortal and insisted that she returned to the otherworld, the two married. Some stories say that Cumhaill abducted Muirenn, and her father’s hatred grew so strong towards the warrior that he did everything within his power to destroy him.
Whichever way it was originally told, the king also grew to loathe Cumhaill because of Tadg’s influence. The two worked together to remove him as captain. Their plan was to have the chieftain of the Morna clan, Goll, challenge Cumhaill for this position.
The Baiscne and Morna clans met at Cnucha for the battle. While Baiscne had more warriors, Tadg was using his sorcery to help Goll and the Morna clan. He used his powers to strike fear in Cumhaill, which made him weak during battle. Although he managed to face Goll and take out one of his eyes, he was no match for the magic being used against him. In the end, Lia, Lord of Luachar, saved Goll and killed Cumhaill.
Muirenn was several months pregnant when the battle occurred, and she was certain that she would never see her husband again. As she inherited her father’s gifts, she knew when he was slain; however, she did not waste time grieving because she knew her and her unborn son were in danger.
She hid in a forest known as Sliab Bladma, and here she would give birth to her son, who she named Demna. She fled and left him with two nurses to lead Goll’s warriors away from him as they were killing all remaining people from the Baiscne clan. She would make it to Munster, where she would eventually marry its king.
Years later, Muirenn would return to bring her son, now known as Finn, to Munster, where he would be protected; however, one of the nurses would not let him go as she wanted him to train and take his father’s place as Captain of the Finna. Finn would go on to tackle the training and obstacles to bring back the Baiscne clan and become the captain.