Irony can also sometimes be found in the form of the poem itself. A classic example is Dylan Thomas's "Do Not Go Gentle into That Good Night," which is the most famous example of a villanelle.
The interesting thing is the the villanelle is a French form, based on a lively dance, traditionally used for light subjects! And of course, Thomas's villanelle is about as dark as they come. The form is being used ironically.