Iroh serves as a guide for Korra in the Spirit World. After his physical death, he chooses to leave his body behind and reside permanently in the Spirit World.
In The Legend of Korra, Iroh appears in the episode “A New Spiritual Age” after Korra becomes lost and emotionally overwhelmed in a dark part of the Spirit World. He leads her toward a safer, brighter area and offers tea, food, and advice. His guidance helps Korra understand that emotions can shape the Spirit World, especially for the Avatar, and that fear and anger can make an already dangerous situation worse.
Iroh is not a spirit in the same sense as Raava, Wan Shi Tong, or other supernatural beings native to that realm. He is the soul of a human who deliberately ascended there, which is why the question's wording can be slightly imprecise. Nevertheless, the story presents him as a resident spirit and a mentor-like guide.
He later assists Tenzin, Kya, and Bumi during their search for Jinora. His role also continues the guidance he provided to Zuko in Avatar: The Last Airbender.