One definition of irony is when a word is used to mean the opposite of what it usually means. The title of this short story definitely fits this definition of irony.
In the story, Pius Ndawula has won a lot of money in the soccer pools. This is where the title of the story comes from. You would think that winning a lottery like this would truly make him a winner.
However, as the story progresses, it seems like he really has not won anything. All his relatives and friends come and start sponging off him. He finds out that all these people are really greedy and don't seem to care about him -- just about his money.
Because of this, it's hard to say Pius is a winner -- he loses most of the money and he finds out all kinds of bad stuff about his relatives and friends. Therefore, calling him the winner is ironic.