[A_Brandybuck]

This is now my long version, I am not very kind with the expressions "meta-reference" and intertextuality' Please forgive my eventual faults. ;-)
I needed much time to elaborate, so my post overlaps with the post of radagastly in one case. ;-)

In Tolkien's own world, there are some parallels to old european myths, especially to nordic myths.

#1 – Edda:
In the Edda (nordic collection of legends, written down in Iceland and Norway hundreds of years ago), there is an epic called Völsungen. In this epic Andwari, a Dwarf, had possessed a magic Ring (Andvarinaut), which was stolen by a God of Fire Loki.
Andwari cursed all future keeper of this Ring. Then there was Fafnir, who killed his father to get this Ring. He took the Ring and guared it in a cave. In this cave the Ring poisoned him and his body, so that he transformed into a dragon or wyvern. After that he was slayn by Sigurd, who then possessed the Ring.

The Völsungen-epic ands shortly after this point, but nearly the same storyline, we find in the Nibelungen-Song. Doubtless it is based on the Völsungen. There is also an older version of the epic, in which the Ring is called Draupnir and was possessed by the godfather Odin. It was said, that this Ring was spawning. Every ninth night, there were 8 Rings more. The root of a great treasure. This Ring was also cursed and went to Fafnir (-> Völsungen), who was (as a wyvern) slayn from Siegfried (= Sigurd).
The Nibelungen-Song continues this story. The Nibelungen-Ring is the root of a great treasure (Nibelungen-treasure) and went to Siegfried, after he had slayn Fafnir.
Then the Ring is, after much trouble about the owning rights, one of the reasons, why Siegfried died. (Short version)

The connection to Tolkien is obvious, a cursed Ring, who poisoned his bearer gives it also by Tolkien. The cave of Fafnir reminds of Gollum's cave and changing. The idea of a Ring being the fundament of a great treasure is similar to Tolkien, where the Seven Dwarven-Rings are said to be the fundament of the Dwarven-treasure.

There are also some similarities of names in the Edda. We find lots of names in it, which are also in The Hobbit and Lord of the Rings. Tolkien himself said, that he took them out of the Edda:
There is the question of nomenclature. The dwarf-names, and the wizard's, are from the Elder Edda. (Letter 25)

Thus there is much intertextuality in Tolkien's works.

#2 – Atlantis
The case of Atlantis is more or less equal. A high civilization lives on an island. This civilization is really far advanced in technology. Both islands will be swallowed by the sea. Tolkien himself said, that the Downfall of Númenor[is] a special variety of the Atlantis tradition (Letter 154) and that he has what some might call an Atlantis complex (Letter 163). He also dreamed of the ineluctable Wave, either coming out of the quiet sea, or coming in towering over the green inlands (Letter 257).
I think that here is a point, where we could call it nearly a meta-reference. Of the story of Atlantis is not much known, save there details, which Platon wrote down, but he heard only rumours. Tolkien took the story of Atlantis and put it into his own context. He wrote in Letter 276:
N[umenor] is my personal alteration of the Atlantis myth and/or tradition, and accommodation of it to my general mythology."

#3 – Beowulf and Kalevala
Sadly I haven't read them, so I can't give you any parallels concerning them. I only have some quotes from Tolkien.
Beowulf:
Letter 25:
Beowulf is among my most valued sources; though it was not consiously present to my mind in the process of writing, in which the episode of the theft arose naturally (and almost inevitably) from the circumstances.
Beowulf influenced Tolkien very strong, but I can't say how strong, I don't read it. There are many quotes in his Letters concerning Beowulf.
Kalevala:
This Finnish-National-epic has influenced Tolkien's world also very strong (maybe much stronger).
He was immensely attracted by something in the air of the Kalevala,[...] But the legendarium, of which the Triology is part (the conclusion, was in an attempt to reorganize some of the Kalevala, especially the tale of Kullervo the hapless, into a form of my own. (Letter 163)
We recover the story of the Children of Hurin in the tale of Kullervo, [i]the tragic tale [...] of which is Turin the hero: a figure that might be said [by people who like that sort of thing, though it is not very useful) to be derived from elements in Sigurd the Volsung [-> Edda], Oedipus and the Finnish Kullervo.

There are much more things, that he took out of other things (especially the languages, which I let out).

The central point of his intention is, that he wanted to write a collection of myths, legends or something similar for his England. He wanted to give England an own mythology, because England had no mythology, while other peoples around England have some great legends (Can be found in Letter 131 and indirectly in Book of Lost Tales).
My personal opinion is, that at the beginning of his writings, he took consciously myths from other nations (-> Kullervo) and put them into his own context. That is a wide spread thing. We only need to look to the Rings and the Edda. Parts of stories became reused in other legends in another context. He wrote his mythology in the style of the other mythologies, which had impressed him much. So he used while he was writing known schemas. He worked them in. This was also I think consciously (sometimes more, sometimes less). He mixed the contents of them to create his world (using intertextuality). Atlantis – Numenor was a special case, which can be traced back to his complex.
With his process of writing, his world was growing and changed from a mythology, which was dedicated to England, to an own world with own schemas. The old schemas, which he had adopted, were fading and replaced with his own schemas. Everyone, who has read the Book of Lost Tales could surely confirm this fact. The similarities to the nordic myths, which can be found in the actual status could be traced back to the beginning of his writing.