What is "birog"

Biróg, in Irish mythology, is the name of druidess of the Tuatha De Danann. A folktale recorded by John O'Donovan in 1835 relates how the Fomorian warrior Balor, to frustrate a prophecy that he would be killed by his own grandson, imprisons his only daughter Eithne in the tower of Tory Island, away from any contact with men. But Biróg helps a man called Mac Cinnfhaelaidh, whose magical cow Balor stole, to gain access to the tower and seduce her. Eithne gives birth to triplets, but Balor gathers them up in a sheet and sends a messenger to drown them in a whirlpool. The messenger drowns two of the babies, but unwittingly drops one in the harbour, where he is rescued by Biróg. She takes the child back to his father, who gives him to his brother, Gavida the smith, in fosterage. The boy grows up to kill Balor. By comparison with texts like Cath Maige Tuired and the Lebor Gabála Érenn, the unnamed boy is evidently Lugh, and his father, Mac Cinnfhaelaidh, is a stand-in for Lugh's father Cian.