From the older Gaelic form
Derdriu, meaning unknown, possibly derived from Old Irish
der meaning "daughter". 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).