At Fantastic Fest, the largest genre film festival in the United States, you tend to get two types of films. First, there’s the wacky, chimerical, excess-for-the-sake-of-it, ratings-be-damned insanity. Then you get the very pensive, David Fincher- or David Lynch-type narratives. A Dark Song, the debut effort from Irish film director Liam Gavin, is very much the latter. An expertly constructed film, and one that moves at a snail’s pace, asks you to not just have the patience of the main character, Sophia (Catherine Walker), but also have faith that the payoff is worth all the prep. A Dark Song is a claustrophobic picture about a woman who, following the traumatic loss of her son, has…