The full title of his essay was A Modest Proposal for Preventing the Children of poor People in Ireland, from being a Burden to their Parents or Country; and for making them beneficial to the Publick, which in itself should have told most mildly intelligent people that he was less than serious.
I must say that I have never heard of anyone in the 18th Century taking the line that Swift seriously.
In an advertisement for the essay, published in the Dublin Intelligence in 1729, Swift penned the following comment on his own work: '... wherein the author ... ingeniously advises that one fourth part of the infants under two years old be forthwith fattened, brought to market, and sold for food, reasoning that they will be dainty bits for landlords, who, as they have already devoured most of the parents, seem to have best right to eat up the children.'