The unspeakably sad irony of dehumanizing a people group
The gravity of racism is hard to understand, because for people who believe in the equality of the races, it seems nearly impossible to entertain racist ideas. But it happened, and it's important to talk about that. In the 1960's, even the government institutions were openly hostile to the civil rights movements, because the tradition of white supremacy was so prevalent in America, especially the south, that people were killed and injured just for asserting their equality. The irony, just to be clear, is that the USA formed its government around the profession that all people were created equal, even though those very government entities have been complicit in the continual mistreatment of minority groups.
The irony of progress
The sad reality of time is that it's really easy to believe that the present moment is an accurate representation of the past. In other words, people today are not nearly as racist as people from 60 years ago, and that leads us to wrongly conclude that extreme racism was not commonplace. It was absolutely normal for a long time to treat Black people as animals. The irony of progress is that by fixing the issues, we run the risk of forgetting the problem, so it's important to call the past to mind, as Lewis does.
The irony of Barack Obama's presidency
Again, it's not a good thing that this irony is true, but it's important to recognize. If someone could travel back in time and reassure the civil rights movement that one day because of their hard work and sacrifice, the nation would someday see its first black president, what would a civil rights march have thought of that news? Sadly, all signs pointed to the failure of that movement from their perspective, but it worked. It worked, and their sacrifices caused real changes in the culture.
The irony of violence
Sadly, violence became an option for the government of Montgomery. To choose violence in an issue like civil rights is an ironic choice, and a good case could be made that the open systemic injustice against the civil rights movement was ironically, part of their success. By making racism so obvious and by allowing the racism to show itself through the indefensible use of violence against innocent civilians—it helped to show the world the kind of obvious injustice that had been tolerated for far too long.
The irony of government support
Instead of serving its citizens and their goals, the governments of Montgomery and of Georgia both stood against their own people.