In the recently released indie comedy “ My Old Ass ,” the heroine has a little brother who has a pop culture crush on the actress Saoirse Ronan, such that his bedrooms walls are covered with her photos in a manner usually reserved for boy bands. I think most of us know how he feels. Ronan has held the screen and a lot of moviegoers’ hearts since her Oscar-nominated breakthrough as the regretful adolescent bad seed of “Atonement” (2007), and her roles since then — little women and lady birds, immigrant Irish girls in Brooklyn — have charted the arrival not of a diva but of a thoughtful and protean creative artist growing up in public.
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