Deirdre (Irish pronunciation: [ˈdʲɛɾˠdʲɾʲə]) or Derdriu is the foremost tragic heroine in Irish mythology and probably its best-known figure in modern times. She is often called "Deirdre of the Sorrows." Her story is part of the Ulster Cycle, the best-known stories of pre-Christian Ireland.

Deirdre was the daughter of the royal storyteller Fedlimid mac Daill. Before she was born, Cathbad the chief druid at the court of Conchobar mac Nessa, king of Ulster, prophesied that Fedlimid's daughter would grow up to be very beautiful, but that kings and lords would go to war over her, much blood would be shed because of her, and Ulster's three greatest warriors would be forced into exile for her sake. Hearing this, many urged Fedlimid to kill the baby at birth, but Conchobar, aroused by the description of her future beauty, decided to keep the child for himself. He took Deirdre away from her family and had her brought up in seclusion by Leabharcham, an old woman, and planned to marry her when she was old enough. Deirdre grew up, and one day told Leabharcham that she would love a man with hair the color of the raven, skin as white as snow, and lips as red as blood. Leabharcham told her she knew of such a man-- Naoise, a handsome young warrior, hunter and singer at Conchobar's court. With the collusion of Leabharcham, Deirdre met Naoise. At first the young man wanted nothing to do with her, because it was known that she was destined for the king. But Deirdre shamed him into eloping with her. Accompanied by his fiercely loyal brothers Ardan and Ainnle, the sons of Uisnech, they fled to Scotland. For a while, they lived a happy life there, hunting and fishing and living in beautiful places; one place associated with them is Loch Etive. But the furious, humiliated Conchobar tracked them down.