All throughout the plot of Edith Wharton's The House of Mirth the irony and nostalgia that permeate the story are embodied in the character of Lily Bart.
The nostalgia goes hand in hand with the irony: First, we get to know Lily as a woman with a social and family backgrounds that are filled with knowledge, class, well-breeding, and an infinite knowledge of aesthetics. Lily is a woman rich in culture, etiquette, and good manners. However, fate plays a rough number on Lily: She becomes basically an indigent at a very young age where she, as well as many other society girls, was at an age where engagements were set up as a network that would enrich families and perpetuate family names.
The irony is that Lily, despite of her new life as a socialite with no money, has all the grace and beauty that the rich socialites do not posses. It is for this reason that she is consistently invited to partake from their riches as a guest. The rich need her class. This is the irony: Money does not buy class.
Yet, in comes the nostalgia right after the irony. Lily does miss her former life because that is the life that she was taught to live. She places special emphasis on the day when she is told by her father, right after he loses his money, that she is basically a frivolous brat for asking whether they could get a fresh bouquet of flowers for their luncheon every day. She remembers when she priced the flowers at "just 12 or 11 dollars", and she also remembers with contempt how her father repeated this seemingly low amount of money as if it were the end of the world.
This, added to the humiliation that Lily suffers at the evil ways of Bertha, the nasty proposals from Bertha's husband, and the way she had to sell her reputation for a loan, make up for a very sad and nostalgic as well as ironic situation for Lily. This is what, eventually, leads her to kill herself slowly.