Irma smiles with the happiness of someone that feels themselves loved (even though not knowing how much) even though her eyes show the melancholy of parting.

"A pleasure and an honour that I share gladly." she finally says as she opens the door and looks one last time at him.

(It doesn't matter that two familiar faces may be observing them from afar, Irma's gaze is only for Grantaire at this moment.)

This time, she says it meaning it fully.
"Au revoir, Monsieur Grantaire."