Answer:
The irony is that John Dashwood is accusing Elinor of something that he himself contradicts and does not know.
Elinor's understanding of human nature is related to Mrs. Ferrars' feelings for her son Eduard. Elinor is the one who most understands the nature of Mrs. Ferrars, in relation to the other characters.
Step-by-step explanation:
The text above refers to the moment when John Dashwood and Elinor are talking about the position of Mrs. Ferrars, in relation to her son, Eduard.
Mrs. Ferrars told Eduard that she does not recognize him as a son and that she will do everything she can to prevent him from succeeding in life. Upon learning of this, John Dashwood states that Mrs. Ferrars will be upset if Eduard marries someone she does not approve of. However, Elinor claims that Mrs. Ferrars' behavior reveals her nature towards her son. According to Elinor, this is a nature of total neglect, so if she doesn't consider him her son, she won't care about anything he does.
At that time John Dashwood does not agree with Elinor and accuses her of reasoning based on ignorance of human nature, while Elinor is correct about her observation.