In Irish mythology, Elcmar (also Ecmar, Elcmhaire) was the husband of Boann. He may be Nuada under another name. His name indicates an association with horses.
Elcmar was the steward of the Dagda. His wife was Boann, goddess of the River Boyne, who according to myth developed a great passion for the Dagda. To consumate this union, Dagda sent Elcmar to High King Bres. Unfortunately Boann got pregnant, and to protect the sensibilities of his steward, the Dagda extended time so Boann's pregnancy lasted only one day. Boann named the baby mac Og and gave him to Dagda. Dagda gave the baby to his son Midir to raise as a fosterling.
Elcmar and Oengus mac Og meet after the Dagda passed out the sithens and omitted giving Oengus one. 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 spares his life if Elcmar will give him the Brugh for day and night. For his life Elcmar accepts this.
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 developes 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, and it must be assumed this is because of his daughter's abduction and shaming by Midir. In turn Oengus kills Elcmar for killing Midir.
Elcmar was a man betrayed by fate. His master the Dagda slept with his wife. The Dagda's son Midir makes off with Englec, and Oengus Og kills him for avenging his daughter's shame.
Sources The Wooing of Etain http://www.maryjones.us/ctexts/etain.html
Death Tales of the Tuatha de Danann http://www.shee-eire.com/Magic&Mythology/Myths/TheLeborGabala/Deathtales-tuathaDE/Page1.htm
Metrical Dindshenchas http://www.ucc.ie/celt/published/T106500C.html
