The main irony in "Dusk" has to do with Norman Gortsby's self-deception and subsequent enlightenment. As the story begins he is sitting on a park bench observing the passing parade of humanity and feeling, as the narrator states,
. . . not disinclined to take a certain cynical pleasure in observing and labelling his fellow wanderers as they went their ways in the dark stretches between the lamp lights.
He thinks of them as "the defeated," but he doesn't feel any sympathy. He himself is young and relatively prosperous. He must have a good position with some commercial firm and is relaxing after another day's work. He notices an elderly gentleman on the bench beside him who also looks "defeated" but has no pity for him either.
Gortsby is an urbanite. He has developed a hard shell. Obviously, he has spent a lot of time on park benches studying humanity; otherwise, he wouldn't have so many opinions about people who haunt the dusk. Anyone who looks reasonably prosperous and sits on park benches regularly is bound to be approached by panhandlers and grifters. It is important to understand that he is definitely not a wealthy gentleman of leisure; otherwise, the loss of one sovereign would not be important. It would not be a learning experience, and Gortsby's learning experience is story's whole point and purpose.
Gortsby is self-confident. He is not afraid of strangers. He is not the kind of person to jump up and dash off if a stranger sits beside him and starts a conversation. He will listen patiently and politely to panhandlers' appeals and grifters' hard-luck stories, and even to propositions of damsels in distress--but when it comes to handing over money he is adamant. His philosophy might be expressed simply as: "I have to work and work hard for my money. Why shouldn't you do the same?"
When the young grifter plops down beside him and tells a complicated story about losing his hotel, Gortsby listens attentively. Then when the stranger indicates that he needs a loan, he finds out that he is up against a stone wall.
"Of course," said Gortsby slowly, "the weak point of your story is that you can't produce the soap."
When the grifter leaves in a huff, after realizing that he has been wasting his breath, Gortsby happens to find the wrapped cake of soap on the ground by the bench. This is where the irony comes in. He experiences a change of heart. He feels ashamed of himself for insulting a gentleman in temporary need. He realizes he has become too cynical and skeptical as a result of living and working in the competitive environment of a big city where money is everything.
But after he catches up with the young grifter and gives him a whole sovereign plus the cake of soap he found, he sees an elderly gentleman
. . . poking and peering beneath [the bench] and on all sides of it, and recognized his earlier fellow occupant.
When the old man explains that he lost a cake of soap, Gortsby realizes that he has been a sucker and has looked like a fool. The irony in the story is contained in the fact that Gortsby was right about his original attitude towards humanity when he was cynical, skeptical, and cold-hearted, and that he was wrong when he became soft-hearted, generous, and sympathetic.
Gortsby goes from being hard-hearted to being soft-hearted and back to being even harder-hearted. He has learned not to trust anybody and not to assume other people's burdens. This is the message that Saki, a Tory and a reactionary, wanted to convey.