"A Modest Proposal" is an ironic Essay: the author deliberately writes what he does not mean. What is the real thesis? Is there more then one?
On the surface, Swift mocks the English political/cultural invasion of Ireland. On another level, his "modest" proposal is also a stab at the soulless, utilitarian aspects of enlightenment thinking. If there is a "thesis" for Swift, it likely lies in the idea that human problems cannot be solved by rational thought alone.