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.
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.