Two divers were brought to safety by Mulroy Cliff and Coast Rescue Services at around 9.30pm.
It is understood that two divers were diving in Lough Salt when one of them, believed to be from Inishowen, got into difficulty.
The other diver raised the alarm and the Downings based rescue team and Gardai were despatched to the scene.
The diver who called the emergency services is believed to be an Italian tourist.
At this stage the rescue services believe the Co Donegal man became entangled under water.
A rescue boat launched by Mulroy Clift and Coast however and the volunteer crew was able to bring him to the surface.
He was later transferred to a decompression chamber.