Emer [ˈẽβ̃əɾ], in Scottish Gaelic Eimhir in modern Irish Eimhear or Éimhear (Eimer and Éimear are also used as modern versions, though historically incorrect) daughter of Forgall Monach, is the wife of the hero Cú Chulainn in the Ulster Cycle of Irish mythology. Emer was the queen of Ulster, married to Cú Chulainn.
The Ulstermen searched all over Ireland for a suitable wife for Cú Chulainn, but he would have none but Emer. He visited her at Forgall's house at Lusk, County Dublin, and wooed her by trading cryptic riddles with her. Emer would accept Cú Chulainn 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 Cú Chulainn 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. Cú Chulainn took up the challenge. He learned all the arts of war from Scáthach, and while he was there slept with her rival Aoife, or Aífe, leaving her pregnant.
In the meantime, Forgall offered Emer to Lugaid mac Noís, a king of Munster. However, when he heard that Emer loved Cú Chulainn, Lugaid refused her hand.
Cú Chulainn returned from Scotland fully trained, but Forgall still refused to let him marry Emer. Cú Chulainn 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 Cú Chulainn 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 Cú Chulainn'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 Cú Chulainn had many lovers, Emer's only jealousy came when he was entranced into love with Fand, wife of Manannán mac Lir, the king of the great sea, as recounted in the narrative Serglige Con Culainn ("The Wasting Sickness of Cú Chulainn"). She decided to kill her rival, but when she saw the strength of Fand's love for Cú Chulainn she decided to give him up to her. Fand, touched by Emer's magnanimity, decided to return to her own husband. Manannán shook his cloak between Cú Chulainn and Fand, ensuring the two would never meet again, and Cú Chulainn 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 Cú Chulainn 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.