From http://en.wikipedia.org/wiki/Margaret_Douglas,_Fair_Maid_of_Galloway :

Margaret Douglas, Countess of Douglas known as the Fair Maid of Galloway (died c. 1474) was a Scottish noblewoman, a member of the Black Douglas family towards the end of the family's position as a major force in Scotland.

She was the daughter of Archibald Douglas, 5th Earl of Douglas and Eupheme Graham, daughter of Patrick Graham, Earl of Strathearn and Euphemia Stewart, Countess of Strathearn.

She acquired Galloway when her two brothers (one of who was William Douglas, 6th Earl of Douglas) were murdered at the Black Dinner in Edinburgh Castle.

She married her cousin William Douglas, 8th Earl of Douglas. When he was assassinated she obtained permission to marry his brother, James Douglas, 9th Earl of Douglas, but it is not entirely clear that the marriage ever happened, if it did it must have ended in divorce since they both married again. She then married John Stewart, 1st Earl of Atholl.

She had no children by the Earls of Douglas, but two daughters by the Earl of Atholl. These were Janet, wife of Alexander Gordon, 3rd Earl of Huntly and Elizabeth, wife of Andrew Gray, 2nd Lord Gray. They both had issue by their respective husbands.

Margaret Douglas in fiction

She is a significant character in Black Douglas by Nigel Tranter, which is rather speculative about her relationship with the 8th and 9th Earls of Douglas.
She also appears in Maid Margaret, a 1905 novel by Samuel Rutherford Crockett.

She was the daughter of Archibald Douglas, 5th Earl of Douglas, and Eupheme Graham, daughter of Patrick Graham, Earl of Strathearn and Euphemia Stewart, Countess of Strathearn.

She acquired Galloway when her two brothers (one of whom was William Douglas, 6th Earl of Douglas) were murdered at the Black Dinner in Edinburgh Castle.

She married her cousin William Douglas, 8th Earl of Douglas. When he was assassinated she obtained permission to marry his brother, James Douglas, 9th Earl of Douglas, but it is not entirely clear that the marriage ever happened, if it did it must have ended in divorce since they both married again. She then married John Stewart, 1st Earl of Atholl.

She had no children by the Earls of Douglas, but two daughters by the Earl of Atholl. These were Janet, wife of Alexander Gordon, 3rd Earl of Huntly and Elizabeth, wife of Andrew Gray, 2nd Lord Gray. They both had issue by their respective husbands.

"It is said that [Sir Andrew Leslie, second Baron of Balquhain], during his abode on Bennachie, he made an incursion, accompanied by a large body of retainers, up to Strathdon, and devasted the lands, and carried off the daughter and heiress of the Baron of Cray, chief of the Clan Allan, and who was called the Fair Maid of Strathdon; others say that the lady he carried off was a daughter of Sir James Stewart of Inveravon, also called the Fair Maid of Strathdon. It seems certain that by one or other of these ladies he had a natural son, Malcolm, to whom he gave the lands of Cults, and whose great-great-grandson was John Leslie, Bishop of Ross in the reign of Queen Mary.