What is the irony in A Modest Proposal?

Irony exists when there is a discrepancy between what we expect to happen and what actually happens.  In "A Modest Proposal," it is certainly ironic that, though the speaker purports to have been working on and researching his plan for a long time, his "statistics" regarding babies are wrong.  He says that "a child just born will weigh 12 pounds, and in a solar year, if tolerably nursed, increases to 28 pounds."  Even in this day and age, when mothers are in so much better health than they were during Swift's time, most newborn babies do not weigh twelve pounds.  Furthermore, the speaker says that most of these babies are born to mothers who are forced to beg in order to support themselves (and so probably do not have enough to eat), which makes the likelihood that they have giant, twelve-pound babies that much less.  Similarly, most one-year-old children do not weigh anywhere near twenty-eight pounds.  We would expect someone who claimed to have done his research on a problem to at least have his facts straight.

Likewise, the speaker describes his American friend, a person who originally conceived the germ of this idea, as a "very worthy person, a true lover of his country. . . whose virtues [the speaker] highly esteem[s]."  Again, we would not expect a "worthy" individual, full of wonderful virtues, who truly loves his countrymen and women to propose a scheme in which they sell and eat babies.  Cannibalism is not something we expect from a virtuous person.  

Finally, the speaker discusses the

Vast number of poor people, who are aged, diseased, or maimed; and [he has] been desired to employ [his] thoughts what course may be taken to ease the nation of so grievous an encumbrance.  But [he is] not in the least pain upon that matter, because it is very well known, that they are every day dying, and rotting, by cold and famine, and filth, and vermin, as fast as can be reasonably expected.  And as to the young laborers, they are now in almost as hopeful a condition.  (emphasis mine) 

In other words, then, the speaker talks about the older, poor population, assuring readers that they do not present a long-term concern because they will die soon anyway.  He then says that the younger people among the poor are in "almost as hopeful a condition"—I would hardly describe the condition of "dying and rotting" as a "hopeful condition," would you?  Here, our expectations of how a compassionate and thoughtful person, which is how the narrator presents himself, would not lead us to believe that he would look on "dying and rotting" as a "hopeful condition"—such a comment betrays his true character.