She was the daughter of Domhnall I, Earl of Mar and Helen (or Ellen) of Wales (1246–1295), the illegitimate daughter of Llywelyn ab Iorwerth ("the Great") Prince of Wales; she had previously been the wife of Maol Choluim II, Earl of Fife.
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The current chief of Clan Mar is Margaret of Mar, 31st Countess of Mar who descends from the Earls of Mar, first creation (1404) (as deemed by Act of Parliament in 1885).
As a young judge in one of his early opinions, Kirchoff v. Flynn, 786 F.2d 320 (CA7 1986), a lawsuit over an arrest for feeding pigeons in a park, Easterbrook used such language as "trundled to the squadrol" to describe an arrest; and states of the pigeon-feeder that she "will never be confused with the 30th Earl of Mar, whose hobby was kicking pigeons".
Some days before his execution, however, he wrote to Sir Robert Cecil enclosing a copy of instructions which Essex had prepared for presentation to the Earl of Mar, an ambassador to Elizabeth from James, with the object of so poisoning Mar's mind against Cecil and his friends that Mar might communicate suspicion of them to the queen.
Uilleam of Mar, or Uilleam mac Dhonnchaidh (Anglicized: William, Duncan's son), was perhaps the greatest of the mormaers of Mar ruling from 1244 to 1276, also known as Earl of Mar.