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 Dagda. The child was reared by his half brother safely away from any retribution that Elcmar might desire.