Mercedes is a rebel (Dramatic Irony)
Fairly soon after being introduced to her, the viewer learns that Mercedes, Vidal's housekeeper, does not share his politics, and is working with the rebels to take him down. This accounts for some dramatic irony, as the viewer knows that Mercedes is a double agent, but Vidal and his men initially do not.
Vidal's loyalty to the baby boy (Dramatic Irony)
When Ofelia is first putting the mandrake root concoction under Carmen's bed, Vidal comes in and she must hide. From under the bed, she hears him tell Doctor Ferreiro that if it comes down to it, he ought to save the baby boy and not Carmen. In this moment, the viewer knows that Ofelia has heard this command, but Vidal does not know he is being overheard.
The mandrake works (Dramatic Irony)
Vidal and Carmen discover Ofelia's mandrake concoction under the bed and immediately dismiss it as a strange and superstitious practice. While Ofelia and the viewer know that the mandrake is the key to Carmen's unexpected recovery, Vidal and Carmen have no idea. Carmen dismisses her daughter's strange practice and throws the mandrake root into a crackling fire, thus destroying her own chances of surviving childbirth. This is a particularly devastating moment of dramatic irony.
Ofelia's blood (Situational Irony)
At the end, the Faun tells Ofelia that the only way to unlock the portal and once again become the princess of the underworld is to spill some of her baby brother's blood. She refuses, which would suggest that she is sabotaging her own salvation, and no sooner does she refuse than Vidal appears and kills her by shooting her. The irony is that her own blood is enough to open the portal and transport her into the magical world of the underworld. Thus what seemed like a tragic end becomes the key to Ofelia's transcendence of the mortal world and reunion with the kingdom of the underworld.
