Distressed by marrying while pregnant, she expelled the creature she had swallowed. Soon after, she became pregnant by Sualdam and gave birth to a boy named Sétanta. Although the tales of Cúchulainn's birth are conflicting, a logical explanation for his origins has never been anticipated.

The Pillow Talk
Medb and Ailill, the queen and king of Connacht, were having a conversation in bed about which of them was wealthier. They tallied their possessions and found them to be equal, except that Ailill owned a magnificent bull, Finbennach, originally the calf of one of Medb's cows that had joined Ailill's herd to avoid belonging to a woman. Medb inquired of Mac Roth if there was another bull of equal value in Ireland. There was only one: the Donn Cúailnge in Ulster. Mac Roth was dispatched to borrow the bull for a year, offering a substantial reward. Dáire, the owner, was happy to lend the bull under such generous terms. However, one of the messengers became intoxicated and declared that if the bull were not lent, they would seize it by force. This angered Dáire, who then ordered them to leave.

The Muster of the Connacht Army
Ailill and Medb gathered their army and awaited a favorable omen before marching. The poetess and prophetess Feidelm warned Medb that the army would suffer great losses at the hands of Cúchulainn, repeatedly stating, "I see them bloody. I see them red."

The Army Encounters Cúchulainn
Fergus, placed in charge of the army, deliberately misleads them to buy time for the Ulstermen to recover from their affliction. Sensing the army's approach, Cúchulainn urges his father, Sualdam, to alert the people. To delay the Connacht army, Cúchulainn issues a challenge, but the army bypasses it. He tries again to stall them by placing a forked branch in the river, impaling the heads of four advancing warriors on it, and declaring that they cannot proceed until someone removes the branch using one hand. Fergus rises to the challenge.

The Boyhood Deeds of Cúchulainn
Ailill inquires about Cúchulainn's age, and upon learning he is only seventeen, Medb dismisses him as an inexperienced warrior. Fergus recounts Cúchulainn's childhood feats, including the tale of how he got his name. As a young boy, he killed a ferocious guard dog that attacked him and then protected the owner's property until a replacement could be raised. Thus, he became known as Cú Chulainn, meaning 'the hound of Culann.'

Cúchulainn's Challenge and the First Series of Combats
Cúchulainn blocks the army's path with a log, challenging them to continue only if a warrior can leap over it in a chariot. Reluctantly, Cúchulainn kills his childhood friend Fráech, who was sent to confront him. Fergus clears the oak with his chariot. Cúchulainn defeats and kills all opponents who face him. Meanwhile, the army ravages Ulster. Ailill suspects an affair between Fergus and Medb, so he dispatches his chariot driver to investigate. The driver discovers them sleeping together and takes Fergus's sword from its sheath, returning to Ailill to confirm his suspicions. Ailill appears satisfied, saying: "She is justified. She does it to keep his help on the raid." He instructs the charioteer to safeguard the sword.

Bargain of the Single Combats
Cúchulainn positions himself at the river Cronn, invoking the elements of air, earth, and the river to assist him. Maine, the son of Ailill and Medb, along with thirty horsemen, reach the river, where Cúchulainn kills them all. These killings persist until Fergus and Lugaid mac Nois Allchomaig negotiate terms of combat that favor Cúchulainn.

The Escape of the Bull, the Bargain of the Single Combats, and the Morrigan
Medb embarks with a third of her forces to capture the bull and devastate Ulster. Cúchulainn encounters Buide mac Báin, Medb's cowherd, who has taken the Bull of Cooley. Cúchulainn kills Buide, but Medb's soldiers succeed in bringing the bull back to their camp. Cúchulainn faces numerous warriors in battle. Amidst these challenges, the Morrígan appears disguised as a young woman and offers herself to him. He rejects her, prompting her to vow to obstruct him. He promises to leave her with lasting wounds unless he grants her a blessing. He continues to confront all champions sent against him. When his foster brother Lóch fights him, the Morrígan intervenes three times and is injured each time. Fergus motivates Briciu to provoke the weary Cúchulainn to maintain his fighting spirit. Cúchulainn realizes his need for assistance. He defeats Lóch but is left drained. The Morrígan reappears as an old woman milking a cow. He requests a drink of milk, and she offers him three. With each drink, he blesses her, and she heals. Medb sets up an ambush, but Cúchulainn overcomes the warriors. She proposes her daughter to him, but he cuts the girl's hair and leaves her at a pillar stone.

Respite for Cúchulainn
Laeg notices a distinguished man approaching them. Cúchulainn recognizes him as one of the síde. The man reveals himself to be Cúchulainn's father from the otherworldly hills, Lug mac Ethnenn. Cúchulainn confesses, "My wounds are severe. It is time they were allowed to heal." Lug advises him to rest and attends to his injuries. While Cúchulainn sleeps, the young warriors of Ulster come to aid him but are tragically killed after a valiant fight, much to Cúchulainn's grief. Cúchulainn commands his sickle chariot, described as having 'every angle and corner, front and rear was a tearing place,' and in a frenzy, he charges through the enemy camped against him.

The Single Combat of Ferdia and Cúchulainn and the Fight of Cethern
An Ulster warrior named Oengus arrives and hurls stones at the Connacht army until he is killed. In search of another champion to face Cúchulainn, Medb approaches Fergus. He refuses to fight his foster son. However, he confronts Cúchulainn, asking him to yield a step if he himself gives way at another time when requested by Cúchulainn. Cúchulainn agrees. The fighting continues, with Cúchulainn emerging victorious.

Fíacha mac Fir Febe, an Ulster exile, comes to Cúchulainn's assistance during a battle. Meanwhile, Medb manipulates Cúchulainn's foster brother and closest friend, Ferdia, by falsely accusing Cúchulainn of slandering him. The two engage in combat for four days. Ultimately, Cúchulainn delivers a fatal blow to Ferdia, mourning the loss of his dear friend. The Ulster warrior, Cethern, arrives, attacks the enemy forces, and then retreats to Cúchulainn, detailing his numerous injuries. He kills several doctors until Conchobor's physician, Fingin, assesses his wounds from a safe distance. Fingin informs Cethern that he can either remain still for a year and survive or have three days of strength to fight before dying. Cethern opts for the latter and perishes in battle.

Fintan and the Death of Finnabair
Fintan, Cethern's father, arrives to seek revenge for his son's death. His other son is captured and returned under the condition that Fintan refrains from attacking Ailill's forces until the final battle. Rochobad Rigderg comes to assist Cúchulainn, but Ailill sets a trap using Finnabair as bait. Finnabair has confessed her love for Rochobad to her parents, who agree to let them be together if Rochobad grants them a truce until Conchobor arrives.

The Warning of Súaldaim and the Muster of the Men of Ulster
Súaldaim goes to support his son. Cúchulainn sends him to alert the Ulstermen, urging them to come immediately to retaliate against the invaders. Súaldaim reports that "men are slain, women carried off, and cattle driven away." He trips and is decapitated by his own shield, but even in death, his head continues to issue the warning. Conchobor pledges to defeat the raiders and reclaim their spoils.

Ailill asks Mac Roth if he can see the Ulstermen approaching. Mac Roth describes them advancing like a mist filled with lightning. Upon going out again, he observes the Ulster camp and returns with descriptions of the warriors and their forces. Fergus identifies them for the king. A truce is declared until the following morning. The Morrígan appears to foretell the impending slaughter.

The Battle of the Armies and the Two Bulls
The battle commences. Ailill and Medb plead with Fergus to join the fight and return his sword to him. Fergus cuts through the Ulstermen and faces Conchobor but is held back by Cormac, who urges him to remember his own kin. Cúchulainn, leaving his sickbed, arms himself and confronts Fergus, insisting he honor his promise to retreat before him. Fergus withdraws the Ulster exiles from the battle, followed by the men of Leinster and Munster. Cúchulainn catches up with Medb but chooses to spare her. Fergus remarks that the battle's outcome is predictable when the herd follows a mare.

The next morning, the survivors assemble to witness the battle between the two bulls. The Donn Cúailnge emerges victorious over Finnbennach, scattering parts of Finnbennach's body across the area. The locations where they clash later become known by specific place names, marking significant events. Ultimately, the Donn Cúailnge meets its end at Druim Tairb.