One instance of irony comes early on in Geoffrey Chaucer's long poem The Parlement of Foules. The ironic moment occurs when Chaucer compares all the "newe science" that comes out of "olde bokes" to the "newe corn" that comes from "olde feldes." Such a comparison could be ironic because, in general, producing corn and pursuing literature tend to be thought of as opposite activities. One is generally considered to be high-minded and lofty, while the other is typically portrayed as a physical activity that doesn't demand much august thought.
The corn metaphor takes on additional irony as the speaker moves on to intellectual topics like heaven, hell, nature, and love. The corn comparison seems to subvert the imputably profound aura that tends to surround discussions of the afterlife, nature, and courtship.
When the reader arrives at the part where Chaucer describes the birds, they're likely to find further irony. One could find it ironic that Chaucer writes about these birds as if he's talking about humans. He endows them with human traits as if they, too, are English subjects. Chaucer remarks, "ther sat a quene." Yet he is not talking about a human queen, he's talking about a bird (a very beautiful bird, but a bird nonetheless).
Indeed, one might wonder how a swan can be "jalous" or why a crow has a "voice of care." Jealousy and compassion are human emotions. Fastening human traits to birds could startle or perplex some readers (or it might make them laugh), but that's kind of what irony is supposed to do.