This is actually a super contentious topic that will start huge fights at conferences, but the academic consensus is more or less: no, these texts are not the pure transmission of some ancient oral tradition. In fact, since the 1980s, more and more scholars have tried to demonstrate that the corpus of early medieval Irish literature is actually heavily influenced by the Graeco-Roman Classical tradition and by the Bible as well.
So just to give you some background: in the early to mid 20th century, Irish nationalist scholars took it for granted that the mythological cycles basically told Ireland’s ancient history in a more or less factual and systematic way. They believed that events described in the Ulster Cycle such as the wars between Ulster and the rest of the men of Ireland (which supposedly took place roughly around the birth of Christ) were inarguably historical events, that the characters in them were real historical figures, and that the kinds of social and political structures portrayed in the texts were accurate.
This way of thinking about Irish texts drew from a myriad of weird and unfounded methodologies. The field of Indo-European studies was a pretty new thing, and some Irish historians noted that the Old Irish and Sanskrit words for “king” (gen. ríg and rājā) both preserved a common Indo-European root, which obviously indicated that the ancient political systems of the Indo-Europeans had been faithfully preserved in both countries. The next logical step was to conclude that Irish and Indian societies were super conservative and basically never underwent any changes until pesky foreigners entered the scene. If it was the case that these were super conservative societies that faithfully stuck to the old ways, then the obvious conclusion was that the myths and legends transmitted through the ages must have also never undergone any real changes.
This conception was later linked to contemporary scholarship dealing with the Homeric epics. In the 1950s two historians (Albert Lord and Denys Page) both wrote books which argued that the body of Homeric poetry was transmitted from an ancient oral tradition that had been passed on through the centuries since the actual Trojan War. Because of this, texts like the Iliad could be studied not only as historical sources about the war, but also about political structures and society in really ancient Greece.
So now let’s get back to Ireland – the first cracks in the traditional interpretation of Irish mythical sagas actually began in the 1920s. Rudolf Thurneysen, a Swiss philologist who was an expert in Old Irish, published a huge book where he attempted to date every Irish literary text using historical linguistics (the ambition of early 20th century scholars never ceases to amaze) and ended up categorizing them into the “cycles” that people still use today. Using his philological technique, Thurneysen actually realized that the Táin Bo Cuailgne, Ireland’s national epic, could not have possibly been the simple transmission of some ancient oral tradition. He noted discrepancies in the text’s Irish grammar which indicated that it was actually a composition of two earlier manuscripts from around the 8th century, which had been later compiled and edited in the 11th century.
Not only that, but by noticing the discrepancies in language, he also realized that some sections of the text had actually been written in the 11th century when it was recopied. This revelation was crucially important in that it opened the door for later historians to analyse Irish myths in a new light: Irish scribes were in fact changing the content of the texts and developing & experimenting with literary tropes throughout the early middle ages. This was pretty earth shattering because it opened up a whole new possibility – that Irish literature might have actually been influenced by, or even composed in imitation of, other literary traditions.
This was more forcibly argued by James Carney in the 1960s. He basically stated that we have to stop thinking about Irish myths as being these texts written down by some scribe while a druid or whatever rattles on about ancient cattle raids and kings. These texts were written by literate men in a literate context (monastic libraries and scriptoriums) who as monks were in dialogue with not only the Christian Bible, but the inherited Classical canon as well. He thought that these monks were very concerned with presenting their ancient traditions as literature that was on-par with works like Vergil’s Aeneid and Homer’s Odyssey and that they demonstrably had access to these texts (though it now seems more likely that the Irish only had access to Latin versions of Homer’s works). Consequently, they consciously imitated elements of Classical writing and deliberated inserted them into their texts.
In his most comprehensive example, Carney described how a literary trope called the “watchman motif” which existed in works like Iliad and Thebiad was introduced at some point, and could be demonstrated to have evolved through the centuries as writers played around with the motif. Basically it’s a literary device where instead of a narrator describing a thing or a person, the writer has their characters describe it through dialogue when prompted by a question like “what is it that you see?” He argued that if Irish myths had arrived in their complete form when they were transcribed, there’s no real explanation for the evolution in motifs like this one which can be seen through the centuries. He accepted that there might have been an oral “nucleus” that these stories derived from, but that they were ultimately literate constructions that were heavily influenced by Greek and Roman literature.
Some more recent work has made even more explicit connections between Classical literature and Irish sagas. Brent Miles, for example, has argued that there are tons of episodes found in the Táin Bo Cuailgne where Cu Chullain performs some deeds or is spoken of in a way that almost perfectly imitates deeds that Achilles had performed or things that had been said about him. His reasoning is that Irish authors consciously manipulated their texts in order to demonstrate that Cu Chullain was a hero comparable to Achilles and therefore worthy of respect despite his pagan-ness. He also points out that the use of prophecy before battle in Táin Bo Cuailgne also mirrors how prophetic statements are employed by authors in a number of Classical sagas.
So are these myths really a pure and ancient tradition that had been faithfully preserved by druids before they were committed to writing? Probably not. There was obviously a “core” literary tradition from which they were drawn from, but there’s ample evidence that Irish monastic writers played around with the stories, adding elements and episodes that would have been recognizable and commendable to any reader with a Classical education. The influence of Christianity and the Classical canon on these stories makes any claims that they can actually tell us about the life and history of the ancient Irish very doubtful. But that doesn’t mean that they’re worthless or “corrupted” in any way; they serve as fascinating examples of how learned scholars in early medieval Ireland thought of themselves and their place in a wider world.
Sources:
Binchy, D. A. “The Background of Early Irish Literature”, Studia Hibernica, No. 1 (1961): 7-18.
Carney, James. Studies in Irish Literature and History. Dublin: Mount Salus Press Ltd, 1979.
Miles, Brent. Heroic Saga and Classical Epic in Medieval Ireland. Cambridge: Boydell & Brewer Ltd, 2011.
Thurneysen, Rudolf. Die irische Helden- und Königsage: bis zum siebzehnten Jahrhundert. Halle: Saale, Vorlag von Max Niemeyer, 1921.