The Incorruptible
The central irony of the story is Nostromo is introduced as having one singular defining characteristic. He is universally considered incorruptible. Ironically, the narrative is centered around a narrative that tracks the downfall of Nostromo caused by the fact that he is corrupt.
Mrs. Gould
Mrs. Gould arrives filled with idealistic energy to help her husband run the mine. Ironically, this help will stimulate Mr. Gould to pay more attention on the silver than her. Ultimately, Mrs. Gould's devotion to her husband will be what drives him farther away from her emotionally.
Useless Treasure
Decoud is left alone by Nostromo to safeguard the silver. While Nostromo is back home, Decoud is the only other person on earth aware of the massive value there on the island with him. The irony is that Decoud has access to undreamed-of wealth that is utterly worthless to him under those conditions. This irony drives Decoud to suicide in which he actually uses the silver as mere weight to sink his dead body.
Nostromo's Downfall
Nostromo eventually becomes corrupted by the greed for silver, but it arrives in a deeply ironic way. Upon discovering that four ingots are missing—the bars that Decoud uses in his suicide—Nostromo fears that he will be accused of theft and lose his reputation for incorruptibility. It is therefore ironic that Nostromo gives into the corrupting greed for the silver because he is more afraid of being thought corruptible for a crime he didn't commit than being revealed as guilty for the crime he does commit.
Nostromo's Death
Nostromo's eventual demise is also very ironic. He spends most of the novel growing more paranoid that he will be found as the corruptible thief of the silver. He winds up getting shot to death after accidentally being mistaken for a common trespassing vagabond.
