I think the basic answer is that for some schools of post-modernism (especially those informed by structuralism), there are no answers to be found in the arguments or texts themselves. Instead, what we are looking for is the bricolage or interaction between the texts. Consequently, the search for truth is abandoned and something else is sought.
This is also a modification of two previous philosophers (who some label as post-modern):
- Kierkegaard - who wrote a work called the Concept of Irony as his dissertation. The jury is not done deliberating on its exact meaning (see the argument between Thulstrup and Stewart (see for instance), but I take it to be a critique (in part) of the idea of complete understanding and the need for humor to decipher what's going wrong. (I'm with what I take to be the majority of Kierkegaard scholars in believing Kierkegaard thinks there are right answers even if our ability to hold them is not at the level of full comprehension and the system).
- Heidegger and his idea of destruktion (or if you prefer dekonstruction). The term is often associated with Derrida, because he uses it as well. But the idea for Heidegger is that we need to get away from ontology and ontotheology to understand Being (capitalization intentional) To the degree to which Being is real for Heidegger, this doesn't quite match up with the "only thing we have left is irony" which some postmoderns move towards.