In Irish mythology, Naisi,[1] Noíse or Noisiu (modern spelling: Naoise [ˈn̪ˠiːʃə]) was the nephew of King Conchobar mac Nessa of Ulster, and a son of Uisneach (or Uisliu).
Eventually, Conchobar seeks to get them to return, but a series of unfortunate events (including a geas placed on Fergus mac Róich their protector and escort, as well as Naoise and his brothers' repeated disregard for Deirdre's prophetic warnings) and the treachery of Conchobar brought on by his desire for Deirdre, leads to tragedy.
The tale climaxes with battle at the Red Branch house at Emain Macha, with many dead including Naoise, his brothers Ainle and Ardan, and Conchobar's son Fiacha.
When Deirdre was born, Cathbad the druid named her, and prophesied that she would cause much trouble and strife - on hearing this the assembled lords would have had her killed then, but king Conchobar prevented it and decided to have her brought up in seclusion, and that when she was old enough, he would marry her.
Naisi met with her, and she declared her love for him, and asked to elope, which he agreed to, though he was tardy to carry out, for fear of Conchobar.
They replied they had no want, except the loss of their kinsmen Naisi, Ainle, and Ardan - because of the woman Deirdre.
However Naisi was prohibited from returning to Ireland except with either Cúchulainn, Conall Cernach, or Fergus mac Ross.
Fergus journeyed to Loch Etive in Alba (Scotland), and found the hunting lodges of the Naisi and his two brothers, but there was no recent sign of them.
Fergus became flustered, telling Borrach that he had promised to bring the three back to Emain Macha on the same day that they returned to Ireland.
Fergus decided to stay at the feast and send his own two sons with the returning exiles back to Emain Macha.
Deirdre counseled Naisi to stay at the island of Cuilenn,[n 1] but Fergus's sons (Illann the Fair, and Buinne the Ruthless Red) spoke against it.
She counselled that they should go to Dún Delgan (Dundalk) and stay until Fergus could join them, or seek Cúchulainn as escort, but the brothers were not worried, and did not follow her advice.
Naisi then called for Concobar's chessboard (the Cennchaem) to be brought so they could play - at the same time Conchobar inquired of Deirdre, asking if she was still so beautiful.
Wounded, Illann asked who had done it, and told them he was there to guard Naisi and his brothers - on hearing this Conall was sorry, and killed Fiacha in revenge.