Imposter syndrome
"Be who you are" sounds easier than it is. For Mark, it meant that he had to learn to trust his love for life and art above the opinions of others. The ironic "imposter syndrome" is what comes before, when self-hatred and rejection forces him to try to control himself and repress his blooming attraction to men. The irony of identity is simply that we don't get to control who we really are deep down, but we are judged by others as if we can.
The irony of repressed parents
The alcoholic mother is a great symbol and a sad one. On one hand, she represents the gift of life, the promise of family, and the desire for femininity. However, because of her repression, she also represents the tragic way that people who are oppressed or repressed often become tools to repress other people—even those they are supposed to love most.
The irony of self-love
A child has an ironic discovery to make before he can love himself. The irony is that we are wired by dependence to let others tell us how we should perceive ourselves, but true power comes from being able to accept yourself and then to help others to perceive you more correctly. This irony is shown in the way Mark's parents reject him, and the journey he has to undergo to learn how to love himself after being told not to love himself by the people he trusts most.
The irony of "rebirth"
The "firebird" in the title is Mark and his blossoming self-respect and self-acceptance from the ashes of his parents' rejection of him, and in light of the suffering of life. He is "reborn" like the Messiah that the fundamentalists talk about so often. Ironically, the path toward rebirth was the very opposite of what Mark's society would have advised him.
The irony of religion
Just because Mark criticizes the behavior of certain kinds of religious people doesn't mean he is not religious. His religious attention to art leads him to a transcendental religious experience, so he knows a thing or two about "coming back from the dead." However, he observes that ironically (and tragically) the way people often use religion isn't designed to help them at all, but rather to help them to hurt others. He does this by criticizing Sally's first husband.
