Manx, the third Gaelic language, does not appear in identifiable written form until a.d. 1610. Around 1770, however, John Kelly copied down a piece of oral folklore entitled “Manannán Beg, Mac y Leirr, ny slane coontey yeh Ellan Vannin” (Little Manannán, Son of Leirr, or an account of the Isle of Man). Kelly copied two versions of this ballad [Manx Museum Mss. No. 519 and No. 5072; see also William Cubbon, “Our Literary Treasures,” Isle of Man History and Antiquarian Society Proceedings, Vol. IV]. It would appear that the original composition had been made during the time of Thomas III of Man (1504–1521), for it describes his landing on the island in 1507. The Manx expert J. J. Kneen has confirmed this by recognising fourteen examples of obsolete grammar showing a time far earlier than the eighteenth century for its composition. The ballad is a unique contribution to the literature concerning Manannán, the ocean god. Not long after this time, the Rev. Philip Moore copied down from oral tradition another ballad, “Fin as Oshin,” a copy of which, dated 1789, is now in the British Museum. The ballad is a Manx interpretation of an episode from the story of Fionn Mac Cumhail and his son Ossian.

The Brythonic branch of the Celtic languages began to diverge into Welsh, Cornish, and Breton in the fifth and sixth centuries a.d., and it is in Welsh that our main early Brythonic mythological sagas have survived. However, the Welsh material is nowhere near as extensive or old as the Irish tales and sagas. While Welsh was certainly flourishing as a literary language by the eighth century, apart from fragmentary remains the oldest book wholly in Welsh is the Black Book of Carmarthen, dated to the twelfth century. As to the mythological tales, they are preserved in two Welsh sources: The White Book of Rhydderch (1300–1325) and the Red Book of Hergest (1375–1425). The stories in these two books constitute what is called in Welsh the Mabinogi, or in English, The Four Branches of the Mabinogion. There is evidence that at least three tales originated far earlier than the surviving written forms. “Culwch and Olwen,” for example, reflects a period two centuries earlier in custom, style, and vocabulary.

Like Irish, Welsh produced a greater wealth of manuscript archive during the latter medieval period.

Although Cornish had produced a written form by the tenth century, nothing survives in Cornish that is reflective of the myths and legends of the Mabinogi. We could argue that we have a fragment of a