What features of the trial could be considered ironic in Book the Second, Chapters 1-6?A Tale of Two Cities by Charles Dickens

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. 

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"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.