Apparently there is a moment of sudden realisation at the end of every story in Dubliners, an 'epiphany'. What is the 'epiphany' in "After the Race"?
Jimmy in Joyce's "After the Race" from Dubliners is obsessed with being with and accepted by his European continental friends. He is the paralyzed Dubliner typical of the short story collection who wants something more and wants to be more than he is. He, like Dublin and Ireland as a whole, in Joyce's view, suffers humiliation and exploitation in an attempt to be acknowledged and accepted by the sophisticated European continentals.
His epiphany relates to his early morning realization that he will face feelings of guilt and failure for the money he's lost playing cards. His inexperience and his intoxication lead to his exploitation at the hands of his "friends."
Jimmy is an outsider throughout the story; he's a tag along. In the end, he is, apparently, allowed to tag along because he is an easy mark, someone who is easily exploited. He realizes what he's done, or what he's allowed to be done to him, and he knows he will suffer for it tomorrow. Ironically, when the story closes, tomorrow has already arrived, and he's already feeling guilty. He's been played for a fool, and he knows it.
Not that his knowledge will change anything. He is paralyzed, as are, according to Joyce, all Dubliners.