The men of the Red Branch of Ulster were at a drinking feast in the hall of the Feidlimid the Bard of King Conchobar Mac Nessa when the unborn child in his wife's womb uttered a scream. All the company were amazed at this and Cathbad the Druid offered an explanation saying it meant the child would be of great beauty but would eventually bring about the downfall of the house of Ulster.
After a few months the child was born and was called Deirdre which means "rampageous" on account of her earlier shout. The Ulstermen wanted her killed in the light of the prophecy of Cathbad but King Conchobar intervened asking that she be reared apart from all human contact and when the time came he himself would take her for his wife. He obviously felt that he could forestall the curse with his royal divinity.
So Deirdre was sent to live with a woman called Leborcham to be kept in seclusion until she was of an age to enter the royal court as wife to Conchobar. The years went by and one snowy day when Deirdre was a young woman and extremely beautiful she saw her foster-father flaying a calf in preparation for their dinner, she saw a raven come down to drink the blood on the snow and she remarked that she would like a husband with hair black like the raven, skin white as snow, cheeks red as blood. Leborcham told her that there was indeed such a man and that he was called Naoise son of Uisnech.
Then one day Naoise chanced to pass near her home and Deirdre spied him and went out onto the road pretending to walk by unconscious of his presence. Naoise seeing how pretty she was, declared: 'Fair is the heifer that walks past me!' to which Deirdre replied ' Heifers are wont to be big where there are no bulls!' Then Naoise said knowing of her imminent betrothal to Conchobar Mac Nessa 'You have the best bull in the province the King himself!' Then Deirdre replies 'I would rather go with a young bull such as yourself!' and Naoise mindful of the prophecy draws back and says he cannot do such a thing. Deirdre grabs him by his ears and declares a geas on him saying 'Ears of sorrow and shame shall these be unless you go off with me.'
So poor Naoise had to go off with Deirdre and incurred the wrath of King Conchobar Mac Nessa for eloping with her. His brothers Ardán and Ainnle went with them and they went to Scotland where they lived in exile knowing that if they set foot in Ulster again the Red Branch Army would kill them. They got in to trouble in Scotland unfortunately as a steward of the king there spied Deirdre as she lay beside Naoise and decided that she would make a fitting wife for the king. And so they had to flee from Scotland to one of the western islands off the coast.
After a few years had gone by however The warriors of Ulster themselves relented and decided that The sons of Uisnech had been exiled for long enough after all they had been friends and comrades in arms. So a group of them persuaded King Conchobar to invite the Sons of Uisnech home. Conchobar's pride was still wounded by the elopement so he asked the advice of Eogan Mac Durthacht, King of Fernmag (Farney in Co. Monaghan) who concocted a scheme whereby They would send the honourable Fergus Mac Roich as messenger and invite the brothers and Deirdre back but once on Ulster soil again they would murder the brothers and Conchobar could seize Deirdre for himself. Conchobar agreed to this treacherous scheme.
Thus Fergus Mac Roich went as a messenger of peace to Scotland to bring back Deirdre and the Sons of Uisnech, and they agreed to come back as they were tired of being in exile from their home and loved ones. However as soon as they neared their home they were set upon by Eogan Mac Durthacht and his men and the brothers were murdered while Deirdre was seized and given to Conchobar.
Fergus Mac Roich was enraged that his word of honour was broken in such a treacherous manner and he immediately set to destroying the house of Conchobar and himself and some other warriors including Conchobar's own son Cormac who were similarly outraged at the dishonourable conduct of Conchobar left Emain Macha and went down to Connaught to work for Queen Medb and King Ailill.
Deirdre stayed with Conchobar for a year but never smiled or ate enough food or slept enough or even raised her head from her knee. Conchobar was annoyed with her behaviour and asked her who did she hate the most in the world. She said 'Yourself and Eogan Mac Durthacht murderer of Naoise' 'You shall spend a year with Eogan Mac Durthacht then and be like a sheep between two rams,' he joked.
While she was in the chariot with Conchobar and Eogan as they were going to the assembly of Macha, she cast herself out and in so doing hit her head on a rock and shattered her skull, and that was how she died.