The young boy was found with stab wounds in a house in Tubbercurry - while the man was found dead in a shed outside the house.
The Office of the State Pathologist has been notified.
Gardaí say they're investigating all of the circumstances surrounding the incident at a house in a location known as Banada in Tubbercurry yesterday evening.
A nine-year-old boy was found in the house with stab wounds - while a 21-year-old man dead was found in a shed outside.
The young boy was removed from the scene by Air Ambulance to Sligo Regional Hospital where he was pronounced dead a short time later.
The scene is being sealed off and preserved this morning - and the Office of the State Pathologist has been notified.