http://en.wikipedia.org/wiki/Margaret,_Countess_of_Mar

Margaret of Mar (d. c. 1391) was Countess of Mar in her own right.

She was a daughter of Domhnall II of Mar and after the death of her childless brother Thomas became Countess of Mar. She had married William Douglas, 1st Earl of Douglas, who was succeeded by their son, James Douglas, 2nd Earl of Douglas and Earl of Mar and Garioch in right of his mother, and when he fell, leading the Scots at the battle of Otterburn.

She was succeeded by her daughter, Isabel, who became owner of the Earldom of Mar and the Lordship of the Garioch and became the owner the unentailed lands of the House of Douglas.

Francis L Bell writes:

1327 is the commonest birth date quoted in the National Library of Scotland; these can be corroborated by an entry in the "Inverurie and the Earldom of the Garioch", a history of Scottish families in the National Library. The entry quotes the death of Donald ( Domnhall) in August 1332 and the Succession of the Earldom to his son Thomas who died in 1377 passing the title down to Margaret of Mar.