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. During one such encounter in Virginia City, Nevada, Martin was fatally stabbed with a bowie knife during a barfight.
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.