Cormac's Glossary is a very valuable encyclopedia of Irish oral tradition by the tenth-century priest-king of Cashel, Cormac mac Cuilennan (sometimes Ui Cuilennan). It proports--like the dindsenchas--to give the etymology of over 1400 Irish terms, as well as the history of various gods and goddesses. The etymologies are fairly usless in terms of a true etymology of a term, but in terms of understanding Irish folklore in the 10th century, it is priceless.
The Glossary shows how the Irish were able to incorporate and accept their pagan beliefs and still be able to refer to them, while still considering themselves Christians. The entries are for subjects like the four holidays Samhain, Imbolc, Beltane, and Lughnasadh, and magical practices such as imbas fornosai, as well as the origins and meanings of the names of the gods, such as Brigit, the Morrígan, and Manannán mac Lír.
The text itself exists in two recentions: the earlier being the Book of Leinster version, and the later Book of Hy Many version. It is sometimes said to be all that is left of a larger, lost work, the Saltair Chaisil. This is now lost, and believed to have been a collection of manuscripts edited by Cormac.
It has been edited and translated twice, the first by Whitley Stokes in 1862 and 1868, and the second by Kuno Meyer in 1913.