Irreconcilable Differences begins strongly as a human comedy about a nine-year-old who decides to take legal action to divorce her parents. Unfortunately, this premise is soon jettisoned for a rather familiar tale of a marriage turned sour as shown step-by-step. Set in the world of Hollywood writers and filmmakers, the story is also more fun for the cognoscenti than the average filmgoer.On the witness stand the seeds of her dissatisfaction emerge in the three principals’ testimony. It is regrettably an uninspired and improbable device to tell the yarn. Not a great deal of perception emerges. Ryan O’Neal and Shelley Long spark off a nice romantic chemistry but really need a better vehicle to show off their craft.