The irony of fratricide
In the story, one brother betrays the other, and the wronged brother murders the traitor in an act of swift vengeance. This is ironic because they're brothers, but like Cain and Abel, they are so deeply human that their selfish tendencies make them competitors and enemies.
The femme fatale
The feeling of falling in love is a deeply religious feeling, as if God himself had given a gift to you in the other person. That's what makes a character like Mélisande so ironic. By the time the plot has played out, no one has benefitted from her love, least of all her own daughter who probably won't get to live. She represents the opposite of the life giving mother. She represents the selfish failure of a person to live for the good of others and to take life away.
The irony of royalty
It seems that the more power a person has, the more likely they are to make serious errors because of character flaws. Maybe that's what makes French literature so regal in tone, because the aristocratic element deepens the irony of human failures, making them seem epic. Perhaps there is a correlation between power, wealth and evil, but that's not the intent of this story. The fact that everyone is royal is ironically perfect for showing their humanity and complexity.
The irony of the archetype
Just because the story is simple and obvious, that doesn't make the story less powerful. In terms of irony, archetypal stories like this are deeply ironic, because by being less specific about each character, the story becomes so much more powerful and authoritative, because it takes on a universal quality.
The irony of life and death
The novel ends with an ironic image, the dying woman giving birth to a child. She is both giving life and taking it away, because the implication is that the child will die from its premature complications. The very act of sex is biologically necessary to create life, but for Mélisande, it does the opposite.
