The irony about Dan Cody is clearly expressed in the following quote:
"The transactions in Montana copper that made him many timesa millionaire found him physically robust but on the verge ofsoft-mindedness, and, suspecting this, an infinite number of women tried to separate him from his money."
The fact that Cody could have become a multimillionaire from making deals in metals suggests that he must have had a keen intellect, a sharp mind, to have concluded transactions which earned him so much money.
The irony is that the same hard-earned cash is what led to his "soft-mindedness", which means that he could not think as clearly as he used to or lacked the judgment he used to have. This made him an easy target for women who wanted to relieve him of his wealth. This is exactly what Ella Kaye, "the newspaper woman", did. She was more than likely responsible for Cody's untimely demise and she inherited his entire fortune, taking even the twenty-five-thousand dollars that Cody had left Jay.
Further irony lies in the fact that practically the same fate would befall Jay Gatsby later. His passion for Daisy and his pursuit of her eventually led to his death. Daisy, unlike Ella Kaye though, was indirectly responsible for Jay's tragic fate.
The irony is that the lessons Jay Gatz learns from Dan Cody, far from helping him attain his goals, actually serve to seal his fate.
Gatz had already thought up the Gatsby persona before Cody appeared in his yacht, but, in meeting Cody, Gatz felt as if he had finally arrived in the sphere to which he belonged. For Gatz, “resting on his oars and looking up at the railed deck, the yacht represented all the beauty and glamour in the world.” Cody had earned his millions in mining; despite his gift for making the astute business deal, he was nevertheless "on the verge of soft-mindedness," an easy target for fortune-hunting women. Despite his money, Cody never acquired the polish of the truly upper class. He was "the pioneer debauchee, who during one phase of American life brought back to the Eastern seaboard the savage violence of the frontier brothel and saloon." Cody lacked the snobbishness of someone like Tom, which perhaps explains his willingness to take Gatsby aboard his yacht and, eventually, to trust him implicitly.
Gatsby is never able to make his outside appearance match his inner desires. Perhaps his problems—the lesson that Cody was unable to give him—are the "inner desires" themselves. In the end, the difference between Gatsby and Tom, perhaps, is the difference between wanting and having.
Cody, like Jay Gatsby, was a self-made man. Cody made his fortune mining precious metals. Like Gatsby, also, Cody liked living the good life and he lived it with gusto. Cody took Gatsby with him as he sailed in his yacht to many ports, both domestic and foreign. Gatsby learned a lot from Cody, not the least of which was how to live among the rich. Unlike Gatsby, Cody drank a lot and he wasn't as careful as Gatsby. Ironically, Cody intended for Gatsby to inherit $25,000 upon Cody's death, but because he wasn't careful and let a woman named Ella Kay into his life, Gatsby never received any money from Cody. Still, however, Cody taught Gatsby how to live as Jay Gatsby and not Jimmy Gatz.