The most obvious irony in "The Destructors" by Graham Greene concerns the leader of the Wormsley Common gang. At the beginning of the story, the leader is Blackie; however, when Trevor (who prefers to be called "T") takes over, the entire tenor of the gang and its activities changes, or at least intensifies.
One of the things the boys in the gang talk about doing concerns one of the most unusual sights in the poor neighborhood known as Wormsley Common. In the midst of the poverty and the post-war rubble, a two-hundred-year-old house is still standing, though it is clearly not as strong and unshakable as it once was. Its simple presence is remarkable enough, but the fact that it was built by the famous Christopher Wren, the country's (Britain's) most famous architect, sets this house apart. The boys have nicknamed the old man who lives "Old Misery."
Blackie and the boys always want to kind of "stick it" to the rich, and one of the things he and the gang want to do is sneak into Old Misery's house just to see if they can do it. He has no intentions of their stealing or destroying anything. When T suggests that they destroy the place instead, the boys have to be convinced; but eventually they are captivated by the idea, and that is when T takes over the gang.
Irony is a contrast between two things; in this case, the contrast is between what we expect and what happens. Fifteen-year-old Trevor is the son of an architect (albeit an out-of-work one), and we would therefore expect him to have the greatest appreciation for such a grand old structure--especially after he is allowed to see what an architectural wonder it is. Instead, he sees wants to destroy it. Even after he sees firsthand the magnificent stairway and the fine furnishings and finishes, he still wants to destroy it. This unexpected act from a boy who no doubt learned to appreciate fine architecture generally and Christopher Wren specifically is an example of dramatic irony.
A similar example happens during the gang's destruction of the house. For a young hooligan who has no qualms about participating in a gang and routinely committing illegal acts, T is curiously and surprisingly ethical about Old Misery's money. Blackie is the one who assumes the boys will all get a share of the money they find in the old man's mattress, but T mixes that idea promptly:
“We aren’t thieves.... Nobody is going to steal anything from this house.”
It is unexpected to hear such high-minded morality from a criminal who is about to burn money and destroy a man's house, in addition to the lesser crimes surrounding this act. How ironic for a gang member to claim that he is not a thief. No, he is not a thief, but he is a...fill in the blank however you wish. Whatever you say there, it is ironic.
Be sure to check out the excellent eNotes sites below for more analysis of Graham Green and his works.