Ah, this is indeed a complex situation that has been rendered a bit of a confusing mess for non-scholarly audiences. The short answer is no, academically the Túatha Dé Danann are not regarded as actual people who ever existed. The long answer is, no they are not real people, but it took us a while to settle on exactly what they should be considered.

Currently, as discussed by Mark Williams in Ireland's Immortals which represents the current general consensus that has been begrudgingly reached after a particularly volatile period in scholarship in this topic, the Túatha Dé are regarded as literary characters, primarily the product of medieval Irish scribes. Some of these individuals might have their distant origins in pre-Christian religious figures (Midir and Boanann, for instance, are two that Williams suggests are most likely to have been such) while others are flat out the creation of the medieval period. (the character Ler, Williams points out, is the product of later medieval monks no longer realizing that the 'mac Lir' element of Manannán mac Lir was a false patronymic representing his profession as a fisherman. His father is named Allot in older text before this confusion.) Potentially the most important element of William's argument, however, is that he points out that by the time these characters begin to appear in the saga corpus, Ireland had been Christianized for several centuries, if not half a millennium. While there may be fragments of pre-Christian elements in these figures, they are almost impossible to confidently identify.

As a good example of this, as discussed by Jacqueline Borsje in 'Human Sacrifice in Medieval Irish Literature', Irish scribes had entirely lost track of their own pagan past by the 9th century, and in their fabrication of Crom Cruach to serve as an idol for Saint Patrick to smash, likely as a result of the swelling popularity of Saint Martin of Tours (renowned for his destruction of pagan idols) in Patrick's cult center of Armagh, had to rely on descriptions of pre-Christian religions in the Bible.

But, for the Túatha Dé Danann in particular. Previous scholars have suggested that the Túatha Dé Danann were, in fact, historical persons who were re imagined as deities. However, these scholars are people such as Cormac mac Cuilennáin, the (theoretical) author of Sanas Cormaic, who died in the 10^(th) century. Medieval Irish pseudo-history, particularly the 11^(th) century Lébor Gábala Érenn, presents the Túatha Dé Danann as humans, part of a long and windingly complex genealogy going back to Noah. (other texts, such as Immram Brain, propose that they are humans who did not experience Original Sin, not descendants of Adam, though this is never fully explained)

It has been increasingly rejected that we ought to read medieval Irish literature as texts that the 'Church rewrote... to fit with their prescribed narrative'. By the point that we have extant texts, Ireland had been solidly Christian for centuries, and the scholars writing these stories down or authoring them themselves show very considerable love for these stories. There absolutely was doctoring of texts at the time, but it was primarily a matter of contemporary politics. Such as how the Fomori are separated from the Túatha Dé (who become the Túatha Dé Danann after some linguistic confusion and the character Danu is more-or-less invented to solve it) and are transformed into Norse analogies in a period of anti-Norse sentiment (see: John Carey, 'Myth and Mythography in Cath Maige Tuired' and Ó Cathasaigh, 'Cath Maige Tuired as Exemplary Myth'), or how the Ulster Cycle is suspected of being heavily doctored by political rivals. (see: Joan N. Radner, 'Fury Destroys the World’: Historical Strategy in Ireland’s Ulster Epic'.) While the texts are certainly written in a Christian context, it is challenging for us to identify exactly how many, if any, of them were pre-Christian tales originally. We know that many were the product of poets and monks who loved the material, loved these characters, and told stories about them, not some conspiracy attempting to hide the past. It had been hundreds of years since Ireland was Christianized. If they were trying, they were widely unsuccessful for the fact that they wrote it all down when they could have just not have. (this whole discussion is part of a very large academic argument that begins with Kim McCone's Pagan Past and Christian Present which kicked off the aforementioned volatile period)

But, to return again to your question: no, we do not believe the Túatha Dé Danann were historic humans who were mythologized by Irish scribes. Instead, in fact, the Irish scribes argued that the Túatha Dé Danann were historic humans who had been mythologized by their pre-Christian ancestors. Scholarly, it has been a long time since the 10^(th) century, and though it has been a bumpy ride and we had to decipher medieval Irish to get there, but we are fairly confident in our identification of the Túatha Dé Danann as literary characters, the product of political and social situations in medieval Ireland, some of whom were potentially originally inspired by fragmentary remains of pre-Christian figures.

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Works Cited

Borsje, Jaqueline., 'Human Sacrifice in Medieval Irish Literature' in The Strange World of Human Sacrifice (Leuven, Peeters: 2007) 31-54.

Carey, John,. 'Myth and mythography in Cath Maige Tuired' in Studia Celtica 24–25 (1989–1990) 53–69.

Mac Mathúna, Séamus,. 'Immram Brain: Bran’s Journey to the Land of the Women' in Buchreihe der Zeitschrift für celtische Philologie 2 (Tübingen, Max Niemeyer Verlag: 1985).

McCone, Kim R., 'Pagan Past and Christian Present in Early Irish Literature' in Maynooth Monographs 3 (Maynooth, An Sagart: 1990).

Ó Cathasaigh, Tomás., 'Cath Maige Tuired as exemplary myth' in Folia Gadelica: essays presented by former students to R. A. Breatnach on the Occasion of his Retirement from the Professorship of Irish Language and Literature at University College (Cork, Cork, Cork University Press: 1983) 1–19.

Radner, Joan N., ‘Fury Destroys the World’: Historical Strategy in Ireland’s Ulster Epic' in Mankind Quarterly 23/1 (Fall, 1982) 41-60.*

Stokes, Whitley., 'On the Bodleian fragment of Cormac’s Glossary' in Transactions of the Philological Society 22 (1891–4): 149–206.

Williams, Mark., Ireland's Immortals: A History of the Gods of Irish Myth. (Princeton, Princeton University Press: 2016).

*Please Note: Mankind Quarterly is nothing short of monstrous. It is a journal that engages in disgusting pseudoscience attempting to justify factually incorrect information pushing vile theories in support of white supremacy. Why Radner chose to publish a single article, which does not appear to have any connection to the journal's rotten heart, here is utterly unclear and a mystery that must be addressed by the field. The article is the only discussion of the topic, and while I would greatly prefer to cite literally anything but it, it is the only discussion of the topic and is widely regarded in the field as an excellent article, with the context of the journal it was published in, otherwise almost unknown to the field, only recently being brought to the attention of senior scholars. Very clearly a through investigation must be done in regards to the relationship between the journal and Celtic Studies, though for the time being nothing of the sort has been undertaken.