I will answer the question about Maugham's short story. The irony here is primarily situational. The unexpected happens. In trying to avoid his fate, the servant actually fulfills it. He sees Death at the market place, and flees to Samarra. Death explains to the servant's master that he was surprised to see his servant at the market-place this morning when he had an appointment that night with the servant in Samarra. The irony is particularly effective because Death's response surprises the reader with the unexpected, and we know that the servant's fate will be the opposite of what he expects. The irony works to show that there is no escaping fate. When we think we are avoiding it, we actually are working to make it happen. Oedipus Rex is an example of this type of irony.
I'll let another editor respond to the "Tell-Tale Heart."