The native Irish-Gaelic traditions which bespeak a pagan belief system were gathered in the mid-17th century by the antiquary and Counter-Reformation priest John Keating in his Foras feasa ar Éirinn (“A foundation of knowledge about Ireland”), which aimed to vindicate the country’s long and august pre-history against English detractions. Derided by Enlightenment historians as superstitious and credulous, it gained a new lease of life in the Romantic century as a documentation of Gaelic ancestral myths.

Another source which provided material for mythological research was folklore and popular superstition, which, as per the model of Jacob Grimm, could be interpreted as the fragments of older, pre-Christian belief systems: fairies (Sidhe, interpreted as the descendants of a supernatural race of inhabitants prior to the country’s Gaelic settlement), or quarterly seasonal feast days (Imbolc, 1 February; Bealtaine, 1 May; Lughnasa, 1 August; Samhain, 1 November and correlated with Walpurgis Night). Finally, superhuman elements in heroic epic or romance such as the Táin Bó Cuailgne were also seen as mythological survivals.

While the better-documented Irish-Gaelic tradition dominated this approach to Celtic mythology, folkloric and epic/romantic materials were also available from non-Irish Celtic sources: Scottish folk tales, the Welsh Mabinogion and Breton or British balladry and romances – including Tristan and Isolde. With the elaboration of a Celtic philology (following the comparatist Grammatica Celtica by J.C. Zeuss, 1853), early comparative Celtologists like d’Arbois de Jubainville elaborated outlines of Celtic mythology (Le cycle mythologique irlandais et la mythologie celtique, 1884). This model in turn was used by Celtic revivalists in the separate Celtic countries, e.g. T.W. Rolleston’s Celtic myths and legends (1911, reprinted in 1917 as Myths and legends of the Celtic race). Remarkably, this mythology was exported to Spanish Galicia. Keating’s tales of how the migrating Gaels, on their way to take possession of Ireland, dwelt on Spain’s Atlantic coast, proved a powerful inspiration for historians like Manuel Murguía.