The victim was in a house on Long Lane around 11pm last night.
It’s understood the assailant walked into the home, produced a knife and stabbed his victim in the stomach.
Gardaí say the attacker then fled the scene.
Ambulance crews were able to stem bleeding from a life-threatening wound and take the victim to Letterkenny University Hospital nearby.
A motive for the attack hasn’t been established.
However the speedy work of ambulance staff is believed to have saved the victim’s life. He is now expected to make a full recovery from his injuries.