Writer-director John Carney has become something of a cottage industry, making charming Dublin-set films about everyday people who love to make music whether it brings them fame or not. Movies like Once and Sing Street are designed to make you believe in the power of DIY music magic. They may be gentle pictures, but they remind you that just strumming a guitar can have transformative powers. They’re feel-good movies brushed with just the right amount of wistfulness. On the surface, Power Ballad —which Carney co-wrote with Peter McDonald, who also appears in the film—promises more of the same. Paul Rudd stars as Rick, a middle-aged Kansas-city-born rocker who’s been living in Dublin for 15 years: While on tour with his old band, he met and fell for an Irish girl and never looked back.…