During the trial of Charles Darnay for treason in England, the most salient ironic instance is that of situational irony: Lucie Manette and her father, Dr. Manette are witnesses for the prosecution. Innocently, Lucie repeats the events of the night on which she and her father shared with Darnay the Dover coach. The evidence that incriminates Darnay is Lucie's testimony that papers were exchanged between Darnay and another man. Also, she testifies that Darnay told her that he has traveled back and forth between France and England many times, a fact that arouses great suspicion in this most inauspicious time.
In another ironic twist, the testimony against Darnay comes into question because the identification of Darnay as the true suspect becomes dubious.
"Did you ever see anybody very like the prisoner?" Mr. Stryver asks the witness who has claimed that the prisoner and some fellow-plotter took the Dover mail one night to a garrison where he "collected information."
Not so like (the witness said), as that he could be mistaken.
"look well upon that gentleman, my learned friend ther," pointing to him who had tossed the paper over [Carton] "and then look well upon the prisoner. How say you? Are they very like each other?"
Mr. Sydney Carton bears a strong resemblance to Charles Darnay. Many of the on-lookers note the resemblance, as well. After deliberation, the verdict comes back "ACQUITTED." It is, indeed, ironic that Mr. Carton should bear such a strong resemblance to Mr. Darnay.
In addition to these instances of irony, it is ironic later on that Dr. Manette again must testify in Darnay's defense, but in France, instead. And, again later in the narrative, Sydney Carton saves Charles Darnay when he is to be executed as he was to be drawn and quartered in England.