The plot of ‘Giver’ is Jonas a 12 year old boy who thought he loved in a utopian society where everything was perfect. As the novel goes further and further into the story we come to realize he lives in a dystopian society where (SPOILERS) everyone looses their memories.
The irony is that they are smiling with “torn and bleeding hearts”.
The idea it emphasizes is that they are smiling to hide their pain.
The overall message revealed by the irony is that they are in considerable emotional turmoil and faking happiness to mask their distress.