Legendary 6th-century ard ri [high king] of Ireland, a figure in the Cycle of the Kings, and greatgrandson of the putatively historical Niall Noígiallach [of the Nine Hostages], to whom several fanciful tales have attached. He is thought to have sent the coronation stone, Lia Fáil, to his brother Fergus mac Eirc of Dál Riada in Scotland so that he might be crowned sitting on it; Fergus refused to return it. The best-known story about him portrays the romantic sequence of events that leads to his death. One day while Muirchertach is seated alone on his hunting-mound a beautiful damsel in a green mantle approaches, saying that she has been seeking him, and so fills him with love and desire that he declares he would give all of Ireland for one night of passion with her. She agrees under three conditions, that he never utter her name, that the mother of his children never be in her sight, and that the clerics of the house leave whenever she enters it. In accepting her demands Muirchertach asks what her name is so that he may avoid uttering it. Despite her loveliness, the damsel follows the loathsome Cailb in uttering a string of ambiguous but threatening-sounding names, ‘Sigh, Sough, Storm [Ir. Sín], Rough Wind, Winter-night, Cry, Wail, Groan’. To please the damsel, Muirchertach turns his queen and their children out of his residence at [Sídh] Clettig near the Boyne and instead brings in craftsmen and their wives to his drinking-hall. Seeing what has happened, with the beautiful damsel sitting at Muirchertach's right hand, St Cairnech curses his house and his reign. In her defence the damsel says that she believes in God and is a child of Adam and Eve; yet she can perform works of wonder, such as turning the Boyne water into wine or making pigs from ferns. These flow from her for seven nights until the eve of Wednesday after Samain, when foul weather reminds the king of the onset of winter. The damsel responds, ‘I am the Rough Wind … winter night is my time … Sigh and Wind, winternight.’ When Muirchertach speaks of the storm [Sín] raging outside, she responds sharply, asking why he has uttered her forbidden name and telling him that he is now doomed. Muirchertach admits that he thinks he is indeed doomed, as it has been foretold that he will die as he has killed his own grandfather by burning him in his house. His troubled sleep is now filled with visions of drowning and fire. An angry Sín then sets Muirchertach's house ablaze and surrounds it with phantom attackers led by the menacing Tuathal Máelgarb [rough-head]. In a futile attempt to escape Muirchertach climbs into a wine cask, but he drowns even as the fire falls upon his head and burns the upper part of his body.
From A Dictionary of Celtic Mythology in Oxford Reference.