“Knock Firinn is called by the people of the country ‘Knock Dhoinn Firinne,‘ the mountain of Don of Truth. This mountain is very high, and may be seen for several miles round; and when people are desirous to know whether or not any day will rain, they look at the top of Knock Firinn, and if they see a vapour or mist there, they immediately conclude that rain will soon follow; believing that Donn of that mountain and his aerial assistants are collecting the clouds, and that he holds them there for some short time, to warn the people of the approaching rain. As the appearance of mist on the mountain in the morning is considered an infallible sign that that day will be rainy, Donn is called ‘Donn Firinne,‘ Donn of Truth.”
In ‘Fairy Legends and Traditions of the South of Ireland’ by Thomas Crofton Croker (1828).