What is the irony regarding the comparisons made in the third part of the novel?

Part 3 is full of irony and satire. Immortality, for example, turns out not to be as wonderful as many people think. The Struldbrugs are depressed, perhaps because there is no reason to act quickly. They have all the time in the world. Meanwhile, they have plenty of time to see what mortals have done for themselves and their society in their fleeting time alive. Check out the GradeSaver analysis of this section:

http://www.gradesaver.com/gullivers-travels/study-guide/summary-part-iii-a-voyage-to-laputa--and-japan