1) A diminutive of der daughter, girl, "girly", "lass"
3) A diminuative of dér tear, "tear-let".
2) "Troubler" or "troublemaker". From the footnotes of James Stephens' "Deirdre".
5) A vitrified fort at Inverfarigaig on the eastern shore of Loch Ness, Dún Dearduil Deirdre's Fortress.
Deirdre was the daughter of Fedlimid mac Daill, a bard. When she was born, Cathbad the druid prophesied that she would be very beautiful, with twisted yellow tresses and mesmerizing grey-green eyes, but that kings and lords would go to war over her, and Ulster's three greatest warriors would be forced into exile for her sake. Conchobar mac Nessa, king of Ulster, decided to have her brought up in seclusion by Leabharcham, an old woman, 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--the sons of Uisnech. They fled to Scotland, but wherever they went the local king would try to kill Naoise and his brothers so he could have Deirdre. Eventually they ended up on a remote island, where Conchobar tracked them down.
"Deirdre's Lament", drawing by J.H. Bacon, c.1905.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, forced by his personal geis to accept any offer of hospitality. He sent them on to Emain Macha with his son to protect them. After they had arrived, Conchobar sent Leabharcham to spy on Deirdre, to see whether or not she had lost her beauty in her long years of travel. Leabharcham, trying still to protect Deirdre from a marriage to Conchobar, told him she had lost all her beauty. However, Conchobar had sent another spy, Trendhorn, who told him that Deirdre was as beautiful as ever, although not before having his eye put out by a silver chess piece, thrown by Naoise. The next day, Naoise and his brothers, Ardan and Ainle, faced Conchobar outside Emain Macha, aided by a few Red Branch Knights, before Conchobar evoked their oath of loyalty to him and had Deirdre dragged to his side. At this point, Éogan mac Durthacht threw a spear, killing Naoise, and his brothers were killed shortly after. Fergus and his men arrived immediately after this. He 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 some versions of the story, she died of grief.