Dublin, Midsummer: While absent in New York, the celebrated actor Molly Fox has loaned her house to a playwright friend, who is struggling to write a new work. Over the course of this, the longest day of the year, the playwright reflects upon her own life, Molly's, and that of their mutual friend Andrew, whom she has known since university. Why does Molly never celebrate her own birthday, which falls upon this day? What does it mean to be a playwright or an actor? How have their relationships evolved over the course of many years? Molly Fox's Birthday calls into question the ideas that we hold about who we are; and shows how the past informs the present in ways we might never have imagined.