Situational irony occurs when events don't unfold as expected. Normally, we expect great fanfare to herald momentous events or events of great suffering. But often these occur on the sidelines, the poet argues, as people go about their daily lives. People are preoccupied by the mundane rather than the magnificent. One example in the poem, as noted, is the birth of Jesus taking place quietly and unremarked. The main event noted, however, is the story of Icarus, who in Greek myth flew too close to the sun, which melted the wax on his wings so that he plunged into the sea: the story is famous as an example of youthful ambition overreaching itself. In a painting by Breughel, Auden says, the painter captures the situational irony in Icarus's plight. Nobody is impressed with his fall: 

In Breughel's Icarus, for instance: how everything turns away
Quite leisurely from the disaster....

Auden describes the "ploughman" who might have heard the boy's cry and splash but didn't consider it important and the ship whose crew might have seen the boy falling out of the sky but had other business to attend to and "sailed calmly on." 

Two examples of situational irony--events that occur directly in contradiction to the expectation of the readers/audience--occur in the second stanza with the allusion to Brueghel's Icarus, an oil painting, which ironically positions a fatal calamity for Icarus next to the mundane continuance of daily activity for a "ploughman" and for a personified "expensive delicate ship that must have seen" the "white legs disappearing into the green / Water."

W. H. Auden's poem "Musee des Beaux Arts," a small "portrait" in itself, describes the reactions of people to suffering. The first stanza situationally ironic incongruity of children "skating / On a pond at the edge of the wood," while others reverently await "the miraculous birth," precedes the situational irony in the second stanza with the portrayal of mythological Icarus. Icarus, the son of Daedalus, is given wings by his father but defies his father's warning and dares to fly too close to the sun. As the wax holding the wing feathers together melts and Icarus crashes into the sea, the "ploughman" continues to plow his field, completely unaware of this profoundly tragic incident occurring near him. Auden writes that to the ploughman the incident ironically "was not an important failure." 

Another depiction that contains situational irony is the ship's failure to notice "Something amazing, a boy falling out of the sky" as it "Had somewhere to get to and sailed calmly on," oblivious to the incident of this monumental moment.