There are a couple of things about Daisy and Gatsby's meeting that might be considered ironic. One is that Gatsby has been wanting to meet her again for many years and has gone to a tremendous amount of trouble and expense to arrange a meeting, and yet when they finally do meet he doesn't know what to say to her. It is ironic that such a self-reliant, successful man of the world should be so shy. The same is pretty much true of Daisy. She loses her sophistication and poise at this critical moment.
The other thing that seems somewhat ironic about the meeting is that both of them are rich and live in mansions and yet they meet in a little cottage which seems even smaller because Gatsby has filled it up with flowers.