The earliest of these which can be dated are the Irish manuscripts which comprise Celtic mythology. The books of Dun Cow preserved the romances of the Old Gods and heroes of Ireland. The Book of Leinster supplements the books of the Dun Cow. Others include the Book of Ballymote, Yellow Book of Lecan, and Books of Lecan and Lismore. Unfortunately, many now are only fragmented manuscripts.
Scottish manuscripts corroborate the Irish documents.
These two country's stories, added to the Cuchulainn saga, detail the deeds of Finn, Ossian and the Fenians. They contain stories of other characters more ancient than Finn or Cuchulainn, the Tuatha De Danaan, the goddess tribe of the ancient Gaels. Welsh manuscripts embody the same time period as the Irish and Scottish. Within these sacred writings are the ancient triads, the ancient poems of the bards, and the Mabinogion , which is a collection of ancient Welsh mythology fashioned to the then-current Arthurian romances. Tylwyth Teg's Thirteen Books of Mystery are the legends, beliefs, magickal knowledge, and power of Y Tylwyth Teg.