From the older Gaelic form Derdriu, meaning unknown, possibly derived from a Celtic word meaning "woman". This was the name of a tragic character in Irish legend who died of a broken heart after Conchobhar, the king of Ulster, forced her to be his bride and killed her lover Naoise.
It has only been commonly used as a given name since the 20th century, influenced by two plays featuring the character: William Butler Yeats' 'Deirdre' (1907) and J. M. Synge's 'Deirdre of the Sorrows' (1910).