In Irish and Scottish folklore, An Fir Lea, Far Liath, Fear Liath, or The Grey Man, is a dangerous fairy being associated with fog and mist.
Appearance: The appearance of The Grey Man is said to vary. Some describe encountering a dark shadow in the fog or else a fog that appears to be alive. Other people claim the fairy appears as old, bearded man dressed in a cloak made of fog which obscures his body. Still others describe the entity as a hairy bigfoot-like creature swathed in fog.
Lore: According to lore, sightings of The Grey Man are omens of ill-fortune. The Gray Man is said to delight in causing human fatalities with the fog and mist that accompanies him. In olden times the Gray Man was said responsible for many a shipwreck. Today, the fairy might be blamed for horrible car wrecks due to fog.
Powers: The Grey Man has power over fog and mist as well as having the power to sour milk and ruin peat-moss used for kindling (dampening it so that it will not light). The Grey Man is also blamed for potato rot or blight, which he is said to cause with a mere touch. The Grey Man has a knack for causing intense panic in people who sight him.
Defense Against The Grey Man: As a fairy, The Grey Man would fear iron objects and salt. Holy objects and prayer are said to be helpful as well. One can tell that The Grey Man is near for the presence of an unusual fog and of his overpowering earthy or musty smell.
Trivia: The Grey Man is said to be incapable of speech. It is also believed that The Grey Man (An Fir Lea) originated as a pagan weather god associated with fog and mist who was worshipped around 1500 b.c. in the coastal communities in and around Ireland and Scotland. With the arrival of Christianity the former god was demoted to fairy. The Grey Man is believed to be found of specific locations, such as Ben MacDhui, a mountain peak in Scotland, as well as various rock formations or natural bridges, like The Grey Man's path at Fairhead in north Antrim.
A Field Guide To Irish Fairies by Bob Curran