The idea that the irony emphasizes is "we wear the mask," which means they look happy because they wear the mask, but in fact they are not happy.
Irony is one of figurative language which is using words for expressing something that has the <em>opposite</em> meaning of the actual or its literal meaning
The mask cover up their unhappiness be look happiness
"we smile" >< "our cries to thee from tortured souls arise."
"we sing, but oh the clay is vile beneath our feet, and long the mile; but let the world dream otherwise"
Figurative language refers to words or phrases that are meaningful, but not the actual meaning.
The examples of figurative language: Irony, Simile, Metaphor, Personification, Hyperbole, Allusion
Experienced writers incorporate sentence variety into their writing by varying sentence style and structure using a mixture of different sentence structures reduces repetition and adds emphasis to important points in the text