The first name which was given to Ireland was Inis na bhfiodhbhadh, that is to say Island of the woods; and the person who called that name to it was a warrior of the people of Nin, son of Bel, who came to spy out Ireland, and on his coming thither he found it to be all one forest-wood, except Magh-n-ealta alone. Three times, indeed, Ireland was one continuous wood, according to this old saying, which is in the ancient record: ‘Three times Eire put three coverings and three barenesses off her.’Geoffrey Keating, Foras Feasa ar Éirinn
Geoffrey Keating, a learned Irish priest and poet who lived in the 17th Century, wrote his celebrated book on Irish History in the year 1634. What is remarkable about it is that it includes many citation and sources. It’s a treasure of old Irish lore. You can find it here.