Gabriel's stepson John is the fulfillment of the prophecy (situational irony)
The bitter irony that spurs a good deal of Gabriel's animosity toward John is that John--not Gabriel's sons by blood, but the illegitimate son of Gabriel's wife--seems to be the fulfillment of the promise Gabriel depends on to get him through dark days: that he is the chosen one, who will have children who take up his mantle as children of God and fire-and-brimstone reverends themselves. Though John is obviously a good, intelligent, and thoughtful boy, Gabriel's attitude towards him makes John go through some dark spots--yet John comes out the brighter for them.
Both Gabriel's sons by blood are named "Royal," but they don't act like royalty (verbal irony)
Gabriel had told Esther he wanted to name a future son "Royal." By doing so, he was casting himself in the role of King David in the Old Testament, whom in Biblical genealogies is listed as a direct predecessor to Jesus. But the first Royal is killed in a fight, and the second Royal is also a recalcitrant boy who goes looking for trouble and hangs with bad crowds. They don't act like royalty.
Gabriel has committed the same sin that he judges Elizabeth for (situational irony)
Throughout the book, Gabriel tends to cast the blame for his own actions on the sins of others. He blames Elizabeth's affair and John for Royal's refusal to fulfill the prophecy - though the exact same thing happened with the first Royal, who was the product of Gabriel's own affair with Esther.
"Set thine house in order" (situational irony)
For all that this is Gabriel's key verse/mantra, it applies very poorly to his own life. His children - legitimate and illegitimate - hate him; nearly every major character in the book has been negatively affected by Gabriel and consequently has a complicated relationship to him. He continues to struggle with lust. The irony is that he prides himself on living out his calling as a minister of the Lord.
Go Tell it On the Mountain Questions and Answers
The Question and Answer section for Go Tell it On the Mountain is a great
resource to ask questions, find answers, and discuss the novel.