49Jackson, Kenneth Hurlstone. The Oldest Irish Tradition; a Window on the Iron Age. Cambridge: Cambridge UP, 1964. 8.
Ronald Hutton provides more detail: "During the sixth century, Irish paganism seems to have collapsed. The last king to celebrate a feis, the symbolic marriage to a tutelary goddess, was Diarmait Mac Cerbaill at Tara in 560...His death in 565 removed the last figure in Irish history (or semi-history) who might have professed the pagan Celtic religions." (Hutton, Ronald. The Pagan Religions of the Ancient British Isles: Their Nature and Legacy. Oxford, UK: B. Blackwell, 1991. 262-63.)