Fergus mac Roich, a former king of the Ulaid in exile and Medb's lover, was chosen to lead the way for the army. On the
way the army split in two. Ailill led one party, and Medb and Fergus, leading the other, took the opportunity to sleep
together. But Ailill had sent his charioteer, Cuillius, to spy on them, and Cuillius stole Fergus's sword while he was
in flagrante . Ailill decided to forgive Medb on the grounds that she was probably trying to ensure Fergus' loyalty, and kept
Fergus's sword safe but hidden.
Because of a divine curse on the Ulaid, the invasion was opposed only by the teenage Ulaid hero Cu Chulainn, who held up
the army's advance by demanding single combat at fords. Medb and Ailill offered their daughter Findabair in marriage to a
series of heroes as payment for fighting Cú Chulainn, but all were defeated. Nevertheless, they secured the bull. After
Conchobar finally assembled the Ulaid's army, Ailill gave Fergus back his sword and the final battle began. The Connachta
were forced to retreat, but Donn Cúailnge was brought back to Cruachan, where he fought Finnbhennach, killed him, and
died of his wounds.
Ailill managed to keep his promise to Medb to be without jealousy, despite her many lovers, until he saw her bathing in a
lake with Fergus. His blind brother Lugaid Dalléces was nearby, so Ailill told him a hart and a doe were playing in the lake,
and challenged him to kill them with a spear. Lugaid threw the spear and killed Fergus.
Late in life, the Ulaid hero Conall Cernach came to stay with Ailill and Medb, as theirs was the only household in Ireland
that could satisfy his enormous appetite. Medb found Ailill was seeing another woman and demanded Conall kill him,
something he was happy to do in revenge for Fergus. He fled, but the men of the Connachta pursued and killed him at the
ford of Ballyconell, County Cavan.