'Deirdre of the Sorrows' is a character in which mythology?

Deirdre is the foremost tragic heroine in Irish mythology and probably its best-known figure in modern times. She is known by the epithet 'Deirdre of the Sorrows' (Irish: Deirdre an Bhróin). Her story is part of the Ulster Cycle, the best-known stories of pre-Christian Ireland. The Ulster Cycle is one of the four 'cycles' of Irish mythology and legend, along with the Mythological Cycle, the Fianna Cycle and the Kings' Cycle.

The story describes a Druid’s foretelling, at Deirdre’s birth, that many men would die on her account. Raised in seclusion, she grew to be a woman of astonishing beauty. King Conor fell in love with her, but Deirdre fell in love with Noísi, son of Usnech. They eloped and fled to Scotland with Noísi’s two brothers, where they lived idyllically until they were lured back to Ireland by the treachery of Conor. The sons of Usnech were slain, causing revolt and bloodshed in Ulster. Deirdre took her own life by shattering her head against a rock to avoid falling into the hands of Conor. A later version omits the first half of the story and expands the tragic ending by making Deirdre live for a year with Conor, never smiling, before killing herself.

The story was immensely popular in Ireland and Scotland and survived to the 20th century in Scottish oral tradition; its literary influence continued into the early 20th century, when the Anglo-Irish writers, notably William Butler Yeats and John Millington Synge, dramatized the theme.