Meaning: unknown though there are several theories about it. It comes from the older form of the name, Derdriu, which could be from a Celtic word meaning "woman". I've also seen it meaning "sorrowful" or "mysterious".
It could also be from Old Irish deir (says) + draoi (Druid) essentially meaning "says the Druid" since her birth and sorrow was foretold by a druid. It could also be from Old Irish elementsder“daughter” + dér “tear”(drop) essentially meaning "daughter of the sorrow".
Deirdre is a tragic heroine whose beauty was foretold by a druid before she was born, a beauty that would only bring strife and sorrow. King Conchobhar decided to have her raised in seclusion so that, when she came of age, he would marry her, but things clearly didn’t work out that way. Deirdre fell in love with a warrior, Naoise, and eloped with him. Conchobhar wasn’t too pleased with this and eventually tricked the couple into coming back, having Naoise and his brothers killed. Versions of Deirdre’s death differ with some saying she died of grief, or that she killed herself. Either way, it’s definitely a tragic love story.