A modest proposal is a work of satire. As such, there are really two main ideas. The main idea as it would appear if we take the story at face value and the true point of the author.
The main idea of the actual story has to do with decreasing the overpopulation by selling babies as food. Swift suggests that the wealthy purchase the infants of the poor and serve them as a delicacy. He gives specific details which are designed to disgust and enrage the reader.
Of course, this idea is meant to be ludicrous. He gives specific details which are designed to disgust and enrage the reader. The actual point of the story is to draw attention the true issue. The poor were being treated reproachfully. Particularly, the Irish were being oppressed by the plantation system and (often British) landlords. Swift is using this work to make a political point.