I had come to see Jack as not merely the dude of all gangsters, the most active brain in the New York underworld, but as one of the truly new American Irishmen of his day; Horatio Alger out of Finn McCool and Jesse James, shaping the dream that you could grow up in America and shoot your way to glory and riches.
Marcus states this near the beginning of the novel, as he introduces the legend of Jack Diamond. Finn McCool is a legendary hero of Old Ireland, a giant of mythological proportions. Jesse James is a famous outlaw and an American legend. Horatio Alger is a novelist who wrote tales of poor boys succeeding in America by dint of hard work and luck. Here the narrator suggests that Jack is like the hero of an Alger novel and also an amalgam of McCool and James—a hero who succeeds not through hard work and luck, but through violence.