Irony of Love for Brian de Bois-Guilbert
Although Brian de Bois-Guilbert passionately loves Rebecca, an error in communication has resulted in her having been accused of witchcraft. There is to be a trial by combat. But he is an extremely competent warrior and is especially good with a lance. So instead of fighting on her behalf to clear Rebecca's name and save her from her death sentence, he is compelled by a direct order from his military superior to defend against any who might seek to do so. If he fights to the best of his ability and defeats her protector (the severely wounded Wilfred of Ivanhoe), Rebecca will die. If he does not fight to the best of his ability and allows Wilfred to defeat him, he himself will die. Either way he's in a bad situation.
Irony of Anonymity
Robin Hood, appearing as Robert of Locksley, makes an appearance at the archery contest and wins everything. The reader, who is generally familiar with the "Robert Locksley" persona as a Robin Hood pseudonym, knows exactly who he is. However he is disguised, as is Wilfred of Ivanhoe and King Richard, who must ensure that his usurping brother John does not succeed in stealing his throne. Part of the excitement in the archery tournament comes from the rivalry between Locksley and the other archers. By appearing disguised, King Richard is able to accurately determine who is loyal to him and who is not.
Irony of Class
Wamba and Garth, two characters who are technically slaves or "thralls", take great risk on behalf of their masters. Had they simply allowed Cedric to be killed or captured, they could have been free men very early in the story. Yet despite their slave status they feel enough loyalty to Cedric, whom they regard as a rightful leader and their lord, to risk their own lives to help him and Rowena.
The only reason Cedric disinherits Wilfred in the first place is because he's fallen in love with Rowena, a woman who according to Saxon standards is far above his station. Yet compared to the invading Normans, all Saxons are on about the same level. Had it not been for the Norman occupation, Wilfred would have most likely been unsuccessful in his suit.
Irony of Power
Judging from his performance as the Disinherited Knight in the tournament at Ashby, Wilfred is without question a redoubtable knight and a great fighter. He is injured, however, by a freak accident and spends most of the novel helpless, being carried about and cared for by other people. In the last fight against Brian to save Rebecca, Wilfred is barely able to stay on his horse. So the worthiest and best fighter is ironically reduced to a position in which his skills can't help him, and the only thing he can rely on is divine will.
Irony of Religion
Although Prince John and many other characters make free use of anti-Semitic slurs against Rebecca and her father Cedric, they prove themselves to be honest, hardworking, and virtuous people even when their accusers, who happen to be Christian, are not. Brian de Bois-Guilbert is a womanizer who ignores his holy vows, his superior in the Order is an aging ascetic who does not look at the facts of the case before jumping to conclusions, and Prince John is an usurper who is trying to steal his elder brother's throne.
The book is not anti-Christian or against any religion in particular. Many of the protagonists, particularly Wilfred, Cedric, and Rowena, do belong to that faith. However it is unique for its period because it presents people of a minorit religion in a positive way, and it presents a nuanced description of both Christian and Jewish characters.
