The irony of "us and them"
Instead of welcoming the newcomers, the townspeople reject them, hoping to keep the established order of their community. This novel shows through elaborate plot and irony how this attempt fails. It is because they seek to do business as "us and them," making enemies of others. Their tendency to draw lines between them and others doesn't stop with the newcomers either, but instead it acts like a forest fire, spreading into the community to break the people apart from one another. Paranoia is ironically destructive.
The puritanical alcoholic
Walter loves to be in charge, because he feels he has a good sense of right and wrong, but throughout the book, the reader sees him to be quite capricious and angry. His love for power and control comes across as nearly-Puritanical goodness, so that people who trust him give him power, thinking he is helping when he isn't. The novel depicts him as a person with a serious issue with substance abuse, especially alcohol, and yet, he is always on the hunt for a witch, failing to see his own mistakes.
The judgmental destroyer
Walter is an ironic character in another regard as well. He acts as a judge, allegedly with the motivation of keeping the town in tact, but instead, he just destroys everything he touches. When the novel ends, the final brushstroke is an ironic depiction of Walter setting fire to the last structure in town, including the maps that would have served as evidence of their existence. Not only does he destroy the town, he annihilates the town and burns the evidence of its existence.
Instability and growth
There is a broader situational irony on which this plot hangs: As the population rises in England, the instability of the status quo increases until, in pivotal moments like this, there is a reversal. The instability is not caused by growth, but rather, the growth is so explosive that English law-makers institute desperate legislation that undermines the status quo completely. The people who are averse to change are simply beside themselves trying to hold onto something that is gone forever.
Mr. Quill's death
Mr. Quill's death is ironic and symbolic. There is dramatic irony of course, because of how Walter discovers his body, but there is also a symbolic situational irony, because of Mr. Quill's job in town; he is a chartmaker, so he can be seen as a symbolic depiction of order. When he dies, that is the same as saying that the old way of life is gone; old systems of order are not applicable anymore. He is cremated along with his maps.
