Compare the story of Bran’s expedition to Ireland in the Third Branch of the Welsh
Mabinogion to the expedition of the Gunther and Hagen to the Etzel’s hall in the
Niebelunglenlied. They are obviously variants of the same tale, more clearly so in the version in the Icelandic
Thiðrekssaga where Hagan/Högni initiates the final battle by suddenly killing his young nephew at a banquet, just as his counterpart Efnisien does in the
Mabinogion.
Tales and motifs float between cultures, some found world wide.
Tolkien was a Germanic philologist who had studied and taught Old Icelandic.
He generally favored Germanic legends himself, in part because he had less taste for the wilder and madder and more stylized Celtic legends.
The idea of isles of immortal folk that appears in Irish tales seems to be what Tolkien mostly took from the Celts into his mythology. GandalftheGray has posted already about the parallels between Irish immortals and their lands and some of what Tolkien writes.
One might also notice a possible connection between
Oromë who is in
Sindarin Araw from the West (
Annûn) and the supernatural huntsman
Arawn Head of
Annwn who appears in the First Branch of the Welsh
Mabinogion.
Coincidence in the names? Maybe.
Of course Sindarin was mostly based on Welsh in its phonology and style.
The parallels that MogothsBane finds are weak.
The immensely strong but gluttonous and crude
god Dagda is not much like
Manwë. In style the Dagda with his great club seems more like the Norse Thor and some think the Dagda may have been a thunder god also in the days before the Irish pantheon was mostly euhemerized.
Danu is barely mentioned in surviving texts, other than in the phrase
Tuatha Dé Danaan ‘People of the Goddess Danu’. We don’t know enough about her to equate her with any of Tolkien’s
Valar.
Her Welsh cognate Dôn was mother of Gwydion, Gilfaethwy, Gofannon the Smith and Amaethon ‘Ploughman’ apparently the representative gods of the four classes of Celtic society: priests/Druids/bards, aristocratic warriors, craftsmen, commoners.
There is nothing of this in Tolkien.
Bilbo is hardly Taliesin because he writes poetry. All cultures have their poets and Taliesin is not the only
bard in Celtic stories. Genuine surviving works of Taliesin are strring songs, mostly in praise of King Urien.
Beren is hardly Oisin. This is told also of other Celtic and Germanic and French and Greek heroes.
A comparison of
the Eye of Sauron with the eye of Balor has been made by others. But the Norse Óð
inn also had but one eye.
It is not clear that
Sauron did have one eye only or whether Tolkien is using metaphoric language referring to Sauron’s ability to look out on the world and see much that was occurring.
Balor as the greatest of the opponents of the Tuatha Dé Danaan of necessity corresponds to
Melkor. But the clever god Lug who overthrows him is hardly Tolkien’s
Tulkas.
Norse myth provides Loki as the Melkor counterpart, once one of the gods himself and later imprisoned by them.
The Zoroastrian faith gives Angra Mainyu (Ahriman).
Tolkien’s mythology is a little bit from here and a little bit from there, very well mixed with so much of his own imagination that origins of this or that part of it are obscured.
Celtic hero legnd speaks much of chariots and charioteers and Tolkien writes almost nothing about this kind of society. Nor do his Valar live in an otherworld parallel to and coexisting with our own where what are simple
hills in our world are divine
fortresses in the world of the gods.
This parallel world idea is very strong in Irish legend but absent altogether from Tolkien other than a few remarks about the Wraith World. But certainly the Valar don’t live in the Wraith World.
Note that by his own admission, Tolkien was not particularly learned in comparative mythology and legend, though certainly more so than many who read his
books. And he certainly knew less when he first drew up his mythos of the Valar than he later learned.
And Tolkien did not put much of what learning he had into his tales to the point that any reading of the Norse Eddas on even
Beowulf is at all necessary to explain anything in his writings.
Tolkien either changes radically what he has borrowed or he freshly invents. You can’t fill in the gaps in Tolkien by finding versions of the same stories in mythological or legendary texts.