Deirdre or Derdriu is the foremost tragic heroine in Irish mythology. Her story is part of the Ulster Cycle.

Deirdre was the daughter of Fedlimid mac Daill. When she was born, Cathbad the druid prophesied that she would be very beautiful, but that kings and lords would go to war over her. Conchobar, king of Ulster, decided to have her brought up in seclusion, and marry her when she was old enough. However, she met, fell in love with, and eloped with Naoise, a handsome young warrior, hunter and singer, accompanied by his two brothers. They fled to Scotland, but wherever they went the local king would try and kill Naoise and his brothers so he could have Deirdre. Eventually they ended up on a remote island, where Conchobar tracked them down.

He sent Fergus mac Róich to them with a message of safe conduct home, but on the way back to Emain Macha Fergus was waylaid and Conchobar had Naoise and his brothers murdered so he could forcibly marry Deirdre. Fergus was outraged by this betrayal of his word, and went into exile in Connacht, and fought against Ulster for Ailill and Medb in Táin Bó Cúailnge (the Cattle Raid of Cooley).

Frustrated by Deirdre's lack of love for him, Conchobar offered her to Éogan mac Durthacht, the man who'd murdered Naoise. She committed suicide by leaning out of her chariot and dashing her head against a rock. In later versions of the story, she died of grief.

There are two plays based on Deirdre's story: William Butler Yeats's Deirdre (1907) and J.M. Synge's Deirdre of the Sorrows (1910).

The LÉ Deirdre (P20), a ship in the Irish Naval Service (now decommissioned), was named after her.

See also

• Grainne
• Helen of Troy