The most ironic part of The Scarlett Letter is that Dimmesdale remains throughout the story up until the end as a respected, well-liked, admired, highly-followed, trendy, beloved, praised and nearly-adored pastor whose following got bigger and bigger the more emacited, sick-looking, and odd he became.
This is ironic because it is as if Hawthorne is laughing at those die-hard church goers who see right in front of them that their shining light of a leader is obviously going through some very odd and psychologically detrimental issues that are beginning to show physically- and yet- that is precisely what their blindness leads them to believe: That such bipolar exoticism, furor and decay are the product of such a Christian life that he should be admired evermore. This, is the man that abandoned Hester and lied to his flock right on their faces while dying of guilt inside.
With irony being a contrast or incongruity between what is expected to be and what actually happens, perhaps the greatest irony in The Scarlet Letter is in the fact that a religious group of people who left England to come to the colonies of America seeking freedom constructed a prison as their first building. This group of Puritans become the most restrictive of all religious sects, allowing no transgressions. Yet, in England they fought against the restrictions put upon them.
In an effort to "purify" their church of the corruption and excessiveness of the Anglican Church from which they broke, Puritans allowed no disgraceful celebrations, no ornateness or pagan-like colors no drinking of liquor. Yet, when Hester and Pearl arrive at Governor Bellingham's mansion in Chapter VIII, there sits upon a table
--in token that the sentiment of old English hospitality has not been left behind--stood a large pewter tankard, at the bottom of which , had Hester of Pearl peeped into it, they might have seen the frothy remnant of a recent draught of ale.
In addition, contrary to the Puritan demand for simplicity, the broken glass of the windows admit much light and the front of the edifice
glittered and sparkled as if diamonds had been flung against it by the double handful. The brillancy might have befitted Aladdin's palace, rather than the masion of a grave old Puritan ruler. It was further decorated with strange and seemingly cabalistic figures and diagrams....
The furniture of the governor is "elaborately carved" and a serving-man wears a blue coat, the "customary garb...in the old hereditary halls of England."
Another great irony exists in the punishment of Hester as contrasted to the condition of the Reverend Dimmesdale. While Hester is publicly humiliated upon the scaffold and made to wear the scarlet A of an adultress, she suffers less that Arthur Dimmesdale whose secret sin is not visible to the townspeople, but tortures his soul to the point that his health is ruined and his body makes manifest his inner A. By having her sin exposed, Hester is able to make reparations for her sin through good deeds, helping the aged and ill. Her redemption is contrary to Puritan doctrine that states that faith, not good works, are what save people. Yet, the town recognizes the goodness in Hester, referring to the A as meaning Angel and Able.
Indeed, throughout Hawthorne's novel, there are many, many examples of dramatic and situational irony. For instance, the townspeople call upon Roger Chillingworth to heal their dear minister when it is this sinister man who has told Hester, "He will be mine," and intends to destroy Dimmesdale. When Hester casts aside her scarlet letter and it falls in the brook, Pearl cries and will not cross the brook until Hester resumes her wearing of it in Chapter XIX. In another example, at the end of Hawthorne's novel, even after achieving freedom and peace in England, Hester returns to the colony and her former home. Once there, Hester bends down, picks up the scarlet letter, and replaces it upon her bosom.
One example of irony is the fact that a rose, marked by both the thorn and the color red, identifies the location of a woman being punished for a moment of passionate love. This is ironic because love (often demonstrated by the giving of red roses in particular) should be pleasing. It turns out that the twist here is these moments of love resulted in a life of the pain of a thorn for one woman.
Another example of irony would be that in most societies, a man who took a woman inappropriately would be marked as an offender. Here, the woman wouldn't give him up. What's even more ironic about this man is that he is not a regular criminal, he was a Reverend who has the job of instructing others in moral righteousness.