Iin the paragraph 21, the author is trying to sell his big idea by suggesting that it would be a useful way of reducing the number of "Roman Catholics" in Ireland. If poor Catholics can breed children for meat, then that will mean far fewer Catholics in the country. Swift is satirizing the contempt "Irish Catholics", both for poverty and their religion.
In the following paragraphs, the author argues that breeding the children for meat will also give poor Irish Catholics something valuable, and it was a highly-prized commodity, so they can sell on the open market. In turn, this will benefit their Protestant landlords.
The main object of the satire here is the class of the Protestant landlords. They squeezed exorbitant rents out of their tenants and reduced them to the regular seizure of their goods when they could not afford to pay. Hope this helps.
