The question of dressing for Iris should be considered with upmost care. You do want to look good for them. A marinière is the obvious choice, but this is a fashion blog, so we don´t do obvious here. Femme Maison´s striped dress will work beautifully. It´s all about the mood, darlings.
Your outfit is not complete without a superb straw hat. Mandatory, darlings. Mühlbauer has the chicest ones. Try them on, and take your time. Choosing a hat is serious business, and should include a glass of champagne and some macarons. My choice would be the Bert Alessa H. Everyone can have une tête à chapeau.