Irony, in this sense, is usually contained within the story, a result of several kinds of narrative operations coming to a crucial moment of ironic intensity that then drives the story into directions one , with hope, didn't anticipate. Post modern writers start off with the intent of being post modern from the start, and rather than have their inventions gear us for a challenge to see the world in a truer light (contrasted against previous schools of lovely language but false conclusions), the project is to debunk the idea of narrative style all together.
Irony is intended to demonstrate some flaws in character's assumptions about the world, a description of the world that emerges contrarily after we've been introduced to the zeitgeist of the fictionalized terrain. Post modern writers are ironists of a different sort, decidedly more acidic and cynical about whether narrative in any form can hone our instincts. It's a tenet of Modernism that in order for writing to be truly contemporary, it must achieve a level of difficulty that allegedly force the reader to reassess their take on experience. Impenetrability was encouraged, so far as the Modernist project encouraged any specific tendency among its early practitioners.
"Make it new" was a chief slogan at the height of the Modernist literary movement, courtesy of Karl Shapiro, and the works, assimilated into academic study, don't comprise the sort of literature that makes for lazy readers. Rather, it's techniques set up the ideal reader, say, "reared in the Modernist style", to grasp the manner and aim of a Postmodern writing, which again, I believe, in it's best expression, is an extension of the Modernist agenda, albeit tweaked about the edges with a bankrupt critical apparatus. The theory cannot keep apace with the actual imaginative writing: sorry, but many theorists seem like bright children adept at taking things apart who cannot quite put them back together in anyway that's useful, meaningful.