Of numerous ironies in Harriet Jacobs's Incidents in the Life of a Slave Girl, I will select three for discussion.
The most striking feature in the early part of the narrative involves Linda's (Jacobs's) family history as she relates it. Few white Americans have realized or acknowledged the extent to which "miscegenation" was entrenched in the slavery system. Even the use of that term, as if it represented some sort of anomaly, was evidence of the hypocritical stance of those who publicly condemned such activity while indulging in it every chance they could. Jacobs tells us that her grandmother was "the daughter of a South Carolina planter who, at his death, left her mother and his three children free." Irony is actually much too neutral a term for the behavior of men who enslave their own children (or anybody, for that matter), but given the overall system, there is a second level of irony in the fact that the slaveowner was generous enough to free them. That the practitioners professed to be religious, as Jacobs points out, while exercising extreme cruelty has been described many times.
Two more things are especially worth commenting on as grimly ironic. A side effect of the "masters" forcing themselves on the young enslaved women was that their "mistresses" developed a savage hatred for these women out of jealousy. One would think these men, at least in the interests of domestic harmony, could have exercised more "prudence," but such was not the case.
Last, Jacobs describes the "muster" of militias, chiefly made up of the poor whites, in the aftermath of Nat Turner's insurrection. The white owner class basically redirected the poor whites' resentment of them toward the black population, and these militiamen reveled in persecuting the enslaved people who, at any pretext, were suspected of disloyalty. It's ironic, of course, that they bought the propaganda instead of turning their anger on the white gentry, who made no efforts to help alleviate the general poverty of the free southern population.