Irony in The Devil and Tom Walker
Irving uses irony in order to criticize the backwards moral makeup of New England society. Usurers like Tom and the corrupt buccaneer, Absalom Crowninshield, are rewarded for their sinful actions with wealth and power.
Tom's surprise is a good example of irony since he has lived his whole life in sin and shouldn't be so startled to see the devil, the very embodiment of sin, in front of him. His surprise also conveys the idea that either Tom has very little self-awareness about his own spiritual self, or he simply doesn't care.
In a perfect display of irony, Tom maintains his miserly attitude at the very end, even so far as to deny that he has profited from his work as a usurer. It is fitting that the devil arrives in this moment at Tom's "invitation" to take him away to hell.
Recall how the buccaneer Absalom Crowninshield was heralded as a respectable and noble man upon his death even though he led a sinful life. It is quite ironic and a little disturbing that this society praises and rewards the actions of people like Tom with wealth and social status. Irving is likely making a satirical commentary on the American obsession with wealth and powerful figures.
The dark and humorous irony in this passage is that normally nobody would willingly wish to sell her soul to the devil, and that the only reason Tom does not go through with the deal is simply to spite his wife rather than any concerns for his own welfare. Greed and spite have made these two characters completely perverse in their actions and motives.
Ironically, these people in this story that get the devil's mark are not outcasts but leaders of society, which suggests how spiritually corrupt society is. The actually sinful Absalom Crowninshield was praised as a pious man at the time of his death despite his vulgar displays of the wealth that he acquired through disreputable means.
In another example of satire, Irving ironically calls Tom a "friend in need." As a usurer, Tom is truly no one's friend because he takes advantage of the situation by exploiting the debtors vulnerability and ruthlessly driving them to bankruptcy.