Which part of this excerpt from “a modest proposal” by jonathan swift is directed against a particular social class in ireland? i grant this food will be somewhat dear, and therefore very proper for landlords, who, as they have already devoured most of the parents, seem to have the best title to the children. infant's flesh will be in season throughout the year, but more plentiful in march, and a little before and after; for we are told by a grave author, an eminent french physician, that fish being a prolifick dyet, there are more children born in roman catholick countries about nine months after lent, the markets will be more glutted than usual, because the number of popish infants, is at least three to one in this kingdom, and therefore it will have one other collateral advantage, by lessening the number of papists among us

In "A Modest Proposal," Jonathan Swift uses irony to show the ridiculous nature of society, the imbalance of wealth and the hunger that it results in. It is a social statement using a most dramatic example: after listing the statistics, Swift weighs the options to solve this hunger, coming to the conclusion that the best option is to have poor couples produce young children to feed the rest of their families, or to sell to the rich in return for money enough to buy themselves food—that's right, he's talking about cannibalism (it's sarcasm, don't worry). In this particular excerpt, he addresses one of the contributors to the issue of hunger, landlords. When he says "landlords...have already devoured most of the parents," he is making commentary on how the price of land leaves little money for food when families are poor. The landlords are so greedy that they would rather have wealth and watch people starve than have less and aid families in need. However, landlords are everywhere, whereas the other social class that he criticizes populates much of Ireland: the Roman Catholics. Note that "Popish" and "Papists" are just other words that refer to Roman Catholics. He claims that during lent, a season meant for fasting and restraint, these hypocritical Catholics are celebrating and indulging in lust, producing children in a time where they, in respect to religious tradition, should not. He is a critic of society and of religion, and this excerpt,  he points out a flaw in the church, while cleverly tying it into his argument that people keep having children. He claims that increased population also increases hunger—when the demand for food increases, so does the price, making food harder to come by for impoverished people. When he says that "...it will have one other collateral advantage, by lessening the number of Papists among us," he means that he is relieved that with his plan, those Roman Catholic children will die, and not continue to populate society. 

"...there are more children born in Roman Catholick countries about nine months after Lent, the markets will be more glutted than usual, because the number of Popish infants, is at least three to one in this kingdom, and therefore it will have one other collateral advantage, by lessening the number of Papists among us,"

hope this helps :)