Darcy's Perspective: Darcy knows Wickham from their shared time in the militia. In contrast to Jane's positive impression, Darcy sees Wickham as untrustworthy and deceitful. He has witnessed Wickham's manipulation and irresponsibility, including Wickham's failed attempt to elope with Georgiana Darcy younger sister.
Why it's Ironic: The contrasting perspectives of Jane and Darcy create an ironic situation. While Jane sees Wickham as a perfect and faultless, Darcy knows that he is far from it. This contrast highlights Jane's innocence and illustrates how people's perceptions of the same individual can be vastly different.
In light Darcy's perspective, the statement that Jane is blindly ignoring the follies of others becomes ironic because her positive impression of Wickham stands in stark contrast to reality. Darcy's knowledge of Wickham's true character suggests that Jane is overlooking his flaws due to her trusting nature.
The irony lies in the mismatch between Jane's perceptions of Wickham based on her limited interactions and Darcy's deeper understanding gained through more experiences with him. This contrast serves as a reminder that appearances can be deceiving and people's judgements may not always align with reality.