What are the conditions of the country at the time?
Conditions in Ireland in 1729 are incredibly bad for the Irish. For many years, wealthy English had been purchasing land in Ireland so that, by the time "A Modest Proposal" is published, 90% of the land in Ireland is owned by the English. These landowners raised rents in order to make more money, rendering it nearly impossible for the Irish tenant farmers to pay their rents and feed and clothe their families (which were typically large). This meant that some people had to abandon the land their families had worked for generations and others had to choose whether to pay rent or feed their children. The number of beggars in the streets increased. Crime went up. It seemed that, as the English got fatter and fatter on their wealth, the Irish got thinner and thinner while they starved; one group seemed to be "eating up" the other, and so Swift capitalizes on this figurative idea, making it literal in his satire.