“A Dark Song,” the moodily intense first feature from the Irish director Liam Gavin, is a striking marriage of acting and atmosphere. Virtually a chamber piece with just two primary characters, the movie dives into the black arts with methodical restraint and escalating unease.

Unfolding almost entirely inside a remote house in the Welsh countryside, the story (also by Mr. Gavin) concerns a grieving mother named Sophia (a marvelous Catherine Walker). Sophia has paid a great deal of money to rent the neglected property. She has also hired a reluctant occultist, Joseph (Steve Oram, perfectly hostile), to guide her through a perilous, monthslong ritual that will allow her to speak with her dead son.

Or will it? As motives shift and resolve weakens, the punishing purification rites and mental preparation begin to take their toll on characters whose flaws were already evident. Toadstools are eaten and blood is supped, but the procedures are neither particularly gory nor pornographically brutal. Focusing instead on psychological strain, Mr. Gavin, working with the cinematographer Cathal Watters, musters multiple shades of smoke and soot into a haunting evocation of a grief so disruptive it will rend the fabric between this world and the next.

“A Dark Song” might be soaked in sorcery, but its true magic lies in its construction. Shot in just 20 days in and around Dublin, the movie has a spare, simple style and rhythms that mimic the characters’ heartbeats: steady and strong; racing and faint; irregular and jumpy. Not until the unexpectedly moving final moments do we realize what Mr. Gavin has been building to all along: not horror or carnage, but a quietly potent, carefully fostered sense of awe.