In the Ulster Cycle of Irish mythology, Emer, or in modern Irish Eimear, daughter of Forgall the Wily, was Cúchulainn's wife.

The Ulstermen searched all over Ireland for a suitable wife for Cúchulainn, but he would have none but Emer. He visted her at Forgall's house at Lusk , County Dublin, and wooed her by trading cryptic riddles with her. Emer would accept Cúchulainn as a husband, but only when his deeds justified it.

However, Forgall was opposed to the match. He came to Ulster in disguise and suggested that Cúchulainn should train in arms with the renowned warrior-woman Scáthach in Scotland, hoping the ordeal would be too much for him and he would be killed. Cúchulainn took up the challenge. He learned all the arts of war from Scáthach, and while he was there slept with her rival Aífe, leaving her pregnant.

In the meantime, Forgall offered Emer to Lugaid mac Nóis, a king of Munster. However, when he heard that Emer loved Cúchulainn, Lugaid refused her hand.

Cúchulainn returned from Scotland fully trained, but Forgall still refused to let him marry Emer. Cúchulainn stormed Forgall's fortress, killing twenty-four of Forgall's men, abducted Emer and stole Forgall's treasure. Forgall himself fell from the ramparts to his death. An ally of Forgall's, Scenn Menn, tried to stop the fleeing couple, but Cúchulainn killed him in single combat at a ford. Having proved his prowess, Emer now agreed to marry him.

Conchobar mac Nessa, the king of Ulster, had the "right of the first night" over all marriages of his subjects. He was afraid of Cúchulainn's reaction if he exercised it in this case, but would lose his authority if he didn't. A solution was found - Conchobar would sleep with Emer on the night of the wedding, but Cathbad the druid would sleep between them.

Though Cúchulainn had many lovers, Emer's only jealousy came when he fell in love with Fand, wife of Manannan mac Lir. She decided to kill her rival, but when she saw the strength of Fand's love for Cúchulainn she decided to give him up to her. Fand, touched by Emer's magnanimity, decided to return to her own husband. Manannan shook his cloak between Cúchulainn and Fand, ensuring the two would never meet again, and Cúchulainn and Emer drank a potion to wipe the whole affair from their memories.

When Aífe's son Connla came to Ireland in search of his father, Emer realised who he was and tried to persuade Cúchulainn not to kill him, but to no avail.

Emer was said to possess the six gifts of womanhood: beauty, a gentle voice, sweet words, wisdom, skill at needlework and chastity.

The LÉ Emer (P21), a ship in the Irish Naval Service, is named after her.