Martin McFly was Seamus McFly's brother.
Irish by birth, Martin arrived in America with his brother Seamus and his sister-in-law Maggie McFly during the early 1880s.
However, Martin had a problem in that he was always trying to prove that he wasn't a chicken, and thus it was easy for others to provoke him into fights. During one such encounter in Virginia City, Nevada, Martin was fatally stabbed with a bowie knife in a saloon. It seems that by the 1980s, his memory had been lost to Seamus and Maggie's descendants for unknown reasons.
In 1885, Seamus and Maggie told "Clint Eastwood" (who was really Marty McFly, Seamus and Maggie's great-great-grandson and Martin's great-great-grandnephew) about Martin and "Eastwood" realized that it was a trait that he shared upon hearing the story. This also revealed that Marty's name wasn't just inspired by his trip to 1955, but also by his ancestor. Remembering Martin's fate and learning that he would be shot dead in a duel with Buford "Mad Dog" Tannen if he faced him with his current attitude, Marty refused to let Buford rile him up.
By not letting his temper get to him, Marty kept a clear head and defeated Buford Tannen with cunning when he was forced to face him anyway. This is also what convinced him to not do a drag race with Douglas Needles, and later not let Needles provoke him into taking part in an illegal deal which in the original 2015 got him fired by their boss Mr. Fujitsu. Marty kept his legacy by naming his son Marty Jr. in 1998, who in turn in named his son Marty III in 2028, and so on.