“Lamb” focuses on the relationship between Brother Michael Lamb (Liam Neeson), a teacher in a Roman Catholic-run institution for troubled boys in Ireland, and 10-year-old Owen Kane (Hugh O’Conor), the school’s most unruly pupil. Dumped into the institution by his abusive mother, Owen is an incorrigible mess who smokes cigarettes, curses, wets his bed, picks fights and has epileptic seizures.