There is insufficient context to determine the exact detail that helps Will realize Buck's identity or the irony related to it in 'A Long Way Down.' Details that typically reveal character identity include distinctive physical traits or shared secrets, and the irony often arises from how something insignificant can prove crucial.
The query pertains to a moment of realization for a character named Will, who comes to know the identity of another character, Buck, within the narrative. The detail or evidence that helps convince Will of Buck's identity is not specified in the provided excerpts, since none of them appear to directly link to 'A Long Way Down' or detail Will's revelation about Buck.
Typically, details that reveal identities in literature can range from physical characteristics to a piece of information only known to the character in question. For instance, a distinctive scar, a familiar mannerism, or knowledge of a secret can often serve as a turning point for character identification. The irony might lie in how something seemingly trivial or overlooked initially could be the key to solving a mystery surrounding a person's identity.
Without the specific context from 'A Long Way Down,' however, a precise answer to this question cannot be confidently provided.