In Irish mythology, Elcmar (also Ecmar, Elcmhaire) was the husband of Boann. He may be Nuada under another name. The mythology of pre-Christian Ireland did not entirely survive the conversion to Christianity, but much of it was preserved, shorn of its religious meanings, in medieval Irish literature, which represents the most extensive and best preserved of all the branches of Celtic mythology. ... In Irish mythology, Boann or Boand (white cow) was the goddess of the river Boyne. ... In Irish mythology, Nuada or Nuadu Airgetlám (Silver Hand) was a king of the Tuatha Dé Danann. ...
Elcmar of the Brug had a wife whose name was Eithne and another name for her was Boand.
Thereupon the Dagda sent Elcmar away on a journey to Bres son of Elatha in Mag nInis, and the Dagda worked great spells upon Elcmar as he set out, that he might not returns betimes (that is, early) and he dispelled the darkness of night for him, and he kept hunger and thirst from him.
Meanwhile the Dagda went in upon Elcmar's wife, and she bore him a son, even Aengus, and the woman was whole of her sickness when Elcmar returned, and he perceived not her offense, that is, that she had lain with the Dagda.
The story goes that the first inhabitants of Brúgh na Bóinne were the goddess Bóann and her husband Nuada Nechtan, otherwise known as Elcmar, "the envious one".
When Elcmar returned, believing that only a day and night had passed, he remained ignorant of events, as the Dagda had taken Aonghus to the rath (fort) of Midhir, where he was trained to be a champion hurler.
Elcmar then called on the Dagda to give his judgement over the situation, and the Dagda ruled that "it is indeed in day and night that all time is spent".