The Irony of Marriage as a Slave
Slaves are not allowed to marry, especially not to white men, so no matter how secure Currer feels in her situation as Jefferson's wife, she's not. In an instant her and her children's lives are entirely transformed for the worse. Her daughters and granddaughters fare no better in their relationships. Despite a mutual love, they are never equal or secure. Green is the perfect example because he simply takes another wife, a white woman this time who sells Clotel and enslaves her daughter.
The Irony of Ellen's Suicide
Ellen is sold into slavery after her parents die because Morton never freed her while he was alive. When her master rapes her, she commits suicide. Rather than become his slave wife, like both her mother and grandmother were before her, she prefers death.
The Irony of Looking White
Currer and certainly her daughters look white. They could "pass" for white, but unfortunately the laws are strictly racist. Because there is still African blood in her, she is still considered a slave.
The Irony of Morton's Abolotionism
Morton is from the North. When he purchases Althesa, he "marries" her. For her that's good enough, but he's lying. He tells her that he's emancipated her and her daughters, but he never does. For all his abolitionist sympathies and helping the girls to pass as white, he doesn't free any of them.
The Irony of Canada
William successfully escapes to Canada. In Canada he's the same person, but he's treated as an equal. They don't believe in the racism of their southern neighbors. All William had to do to receive equal treatment was cross the border.