Lady Bracknell. To speak frankly, I am not in favour of long engagements. They give people the opportunity of finding out each other's character before marriage, which I think is never advisable.
Lady Bracknell says this in response to Gwendolen and Cecily declaring their engagements to Jack and Algernon. What is ironic about this passage?
Her advice to know little about your spouse pre-marriage comes in the wake of learning Algernon and Jack's true identities
Her advice to know little about your spouse pre-marriage comes after learning about Cecily's impending fortune
Her advice to know little about your spouse pre-marriage comes from her own marriage experience
Her advice to know little about your spouse pre-marriage comes from the societal norms exemplified from the times.
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The ironic statement about this passage is "Her advice to know little about your spouse pre-marriage comes after learning about Cecily's impending fortune".

The irony of the passage is Lady Bracknell giving Gwendolen and Cecily advice on relationship that "long term engagement gives people the opportunity of finding out each other's character before marriage" after hearing about Cecily's impending fortune".

Long term engagement gives people the opportunity to know more about their partners character and also learn ways of adapting before marriage. It helps individuals decide if they want to go ahead or opt-out of the relationship before marriage.

Therefore, contrary to Lady Bracknell advice, long term relationship is not bad but should be kept minimal to avoid waste of time.