The irony in the story "A Marker on the Side of the Boat" lies in the fact that the narrator is faced with a seemingly insignificant detail, a marker on the boat, that unexpectedly triggers a profound emotional response and forces them to confront the complexities of their relationships and memories. This contrast between the mundane object and the deep emotional impact it evokes highlights the unpredictability of human emotions and the power of subtle reminders to provoke introspection.