In Irish mythology, Elcmar or Ecmar (modern spelling: Ealcmhar) was the husband of Boann and belonged to the semi-divine race the Tuatha de Danann, the people of Danu. It has been suggested that he is Nuada under another name, or that his name is an epithet for Nechtan the river god. At first glance he appears to be associated horses but there is also a school of thought that says his name means The Evil One.
Elcmar served as chief steward for Dagda, one of the most important and powerful of the Danann. His wife was Boann, goddess of the River Boyne, who according to myth developed a great passion for the Dagda. To consummate this union, Dagda sent Elcmar to High King Bres on an errand around the time of Imbolc. It bears a great similarity to the story of Zeus and Alcmene. Boann, like her Greek counterpart, got pregnant. To protect the sensibilities of his steward and the life of the child, the Dagda extended time so Boann's pregnancy lasted only one day. Boann named the baby Mac Og and gave him to the Dagda. The child was reared by his half brother, Midir, safely away from any retribution that Elcmar might desire.
Elcmar and Oengus mac Og eventually meet when the Dagda passes out the sithens and omits giving one to Oengus. To make up for his mistake, the Dagda tells Oengus how to trick Elcmar out of the Brugh na Boinne. On Samhain Oengus challenges him to a duel and defeats him but spares his life if Elcmar will give him the Brugh for day and night. For his life Elcmar accepts the offer.
The absence of an adjective leads to Elcmar losing the Brugh to Oengus. To placate his steward the Dagda gives him another sithen. There is no love lost between Elcmar and Oengus, and Elcmar is watching the infamous hurling match that led to Midir losing his eye.
Although Boann is not named specifically as the mother of Englec, the daughter of Elcmar, there are no tales of Boann ever leaving her husband. Englec develops an incestual passion for her half brother, although she has never seen him. Slipping away to a hurling match where she expects to see Oengus, she ends up being carried off by other sidhe.
Englec winds up Midir's lover. Elcmar kills Midir afterwards. In turn, Oengus kills Elcmar for killing Midir.