Movies are constantly coming up with reasons to keep lovers apart for long enough to convince audiences that they genuinely belong together, but “Wild Mountain Thyme” may be the first film in which those obstacles are never made clear. Rosemary Muldoon (Emily Blunt) is beautiful. Anthony Reilly (Jamie Dornan) is beautiful. These two Irish neighbors grew up on adjacent farms, and the “once upon a time”-style opening narration — delivered by Anthony’s father, Tony, played by Christopher Walken — makes it all to evident in the opening minutes that these two are destined for one another. And yet, Rosemary and Anthony are not a couple.