The male and female, both in their 40s, were climbing on the Western side of the mountain when the weather conditions changed.
The alarm was raised at 3.45pm and a rescue party from Donegal Mountain Rescue team were dispatched to the scene.
The couple were located at approximately 5.45pm.
However, when the man was walking down the mountain he complained of an upper leg injury and could not walk any further.
The Rescue 118 helicopter was dispatched from Sligo but had to abandon the rescue because of the dense fog.
A second extraction team from DMR was them dispatched to the area and are currently escorting the party down the mountain around 8.30pm.
The man is expected to be taken to Letterkenny General Hospital to have his injury examined.
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