Dickens uses verbal irony to criticize the French aristocracy:
Monseigneur had one truly noble idea of general public business, which was, to let everything go on in its own way; of particular public business, Monseigneur had the other truly noble idea that it must all go his way—tend to his own power and pocket […] The text of his order (altered from the original by only a pronoun, which is not much) ran: ‘The earth and the fulness thereof are mine, saith Monseigneur.’
This passage plays on the two definitions of the word “noble,” which can mean either “morally good” or “aristocratic.” The figure of Monseigneur, who stands in for the entire aristocratic class, holds the “truly noble” belief that the nation will take care of itself as long as he gets everything he wants. In the moral sense, this belief is not noble at all, since it allows for the continued exploitation of the French proletariat. In the aristocratic sense, however, it is the epitome of nobility, since the rich maintain their lavish lifestyles at the expense of the poor.
Dickens uses verbal irony when he describes the prominent place that the guillotine holds in the people’s imagination:
It was the sign of regeneration of the human race. It superseded the Cross. Models of it were worn on breasts from which the Cross was discarded, and it was bowed down to and believed in where the Cross was denied.
Referring to a machine that was invented for mass killing as a “sign of regeneration” is certainly ironic. However, the phrase also contains a kernel of truth, since one of the characteristics of Monseigneur, the synecdochic figure for the aristocracy, is his devastation of the land. By killing off Monseigneur, the revolutionaries hope to usher in a new era of fertility.