All the numerous variations of Otherworld tales now extant in Celtic literature show a common pre-Christian origin, though almost all of them have been coloured by Christian ideas about heaven, hell, and purgatory. From the earliest tales of the over-sea Otherworld type, like those of Bran, Maelduin, and Connla, all of which may go back to the early eighth century as compositions, the christianizing influence is already clearly begun; and in the Voyage of Snedgus and of Mac Riagla, of the late ninth century, this influence predominates. 3 Purely Christian texts of about the same period or later describe the Christian heaven as though it were the pagan Otherworld. Some of these, like the Latin version of the tale of St. Brandan's Voyage, greatly influenced European literature, and probably contributed to the discovery of the New World.
The Fairy-Faith in Celtic Countries, by W.Y. Evans-Wentz. Chapter VI, The Celtic Otherworld.
