Hello, I'm the anon that asked about the Diarmuid and Grainne tale before and you gave such a great answer that I felt like it was safe to maybe send you an ask again? I've read more into the Ulster and Fenian tales and also came across the comparison between Diarmuid/ Grainne and Tristan/Isolde (a lot actually). Does that hold any evidence? Did the Tristan take evolve from Diarmuids story? I was thinking about maybe it up on JSTOR but you're definitely the less intimidating option. Have a great day
So, with the huge caveat that I am not a specialist in fíanaigecht material and also not an expert on Tristan material, this question did come at the only time when I might be able to answer it, because I literally read an article today about this topic. Having said that, I am still extremely not an expert!
First of all, I will say that this has definitely been a topic of discussion in academic scholarship on these texts. What the current consensus is on whether one text is drawing on the other, I can be less confident asserting, because I haven't read a lot of the scholarship on this topic. Although I'm not working on this topic directly, I'm currently looking substantially at the relationship between Arthurian romances and Early Modern* Irish texts (a category to which Diarmait & Gráinne belongs, although I'm working on the Ulster Cycle, so it's not in my corpus), so it's probably something I'll find myself coming back to. It's a very muddy area though -- although Arthurian scholars are often very ready to attribute details to the "Celtic" origins of a story and therefore imply that any similarities mean an Irish or Welsh text is the original blueprint, in this case, the surviving texts are late enough that you get a lot of influence coming back in from French and English sources via the Anglo-Normans and Ireland's general literary contact with the outside world.
When looking for articles on this kind of topic, JSTOR may be able to help, but I tend to find it's a bit limited for Irish material because so many articles and chapters in our field haven't been digitised. Which is a huge disadvantage. However, there are a few ways around this, at least in terms of identifying material (not so much getting access to it). I ran a search for tristan on BILL, the Bibliography of Irish Language & Literature, to see what had been published on the topic recently (but not super recently because BILL tends to be a few years behind with recent publications).
From there, I found Marie-Luise Theuerkauf's 2017 article in The Matter of Britain in Medieval Ireland: reassessments (Irish Texts Society Subsidiary Series 29, ed. John Carey): 'Tristan and early modern Irish romances: James Carney’s Ur-Tristan revisited.' This is a useful place to start. At least, it's where I started while reading today!
In this article, she discusses James Carney's controversial attempts (in the 1950s) to reconstruct an insular "ur-text" of the Tristan story that would have served as the source for later texts. In this study, Carney identifies a number of Irish texts that share motifs with Tristan stories, among them Tóruigheacht Dhiarmada agus Ghráinne. TDG probably dates to the thirteenth or fourteenth centuries, although the earliest manuscript of the text is from the seventeenth century. However, Carney believed that the original story underlying it dated to around 900, and therefore he can count it among his early, pre-Tristan texts.
It's true that there were definitely earlier versions of the story, since the (descriptive) title of one is found in tale lists: "The Elopement of Gráinne with Diarmaid". The text itself is lost, though, so we only have the title. Theuerkauf notes that "this proves that the love triangle story of Diarmaid, Gráinne and Fionn was known at a date anterior to the earliest Continental Tristan texts, we cannot automatically assume that the Aithed Gráinne story in exactly the same way as the Tóruigheacht does." In other words, a lot of the very specific motifs and similarities that the two texts share might be later, so which direction the influence is in becomes very muddy -- does Tristan get it from Diarmaid, or the other way around? Or do they both draw on a shared source? Or are they coincidental/more general folklore motifs from a common stock?
Theuerkauf cautions: "While it may be tempting to focus on the similarities which exist between the Tóruigheacht and Tristan, this focus has tended to lead to an over-simplification of the nature of the source material" and comments that although they're "very much alike in theme, they are often very different in execution or intent". She finishes by concluding that Carney is wrong about most things, if not everything, and introduces for consideration another 15th-century Irish text with close correspondences to the Tristan stories.
Another scholar who has worked on the relationship between the Tristan stories and the Irish material is Raymond J. Cormier. His article "Open Contrast: Tristan and Diarmaid" in Speculum 51/4 is available on JSTOR. I haven't read this one myself yet, but it looks based on Theuerkauf's citations that he tends to be quite critical of the connections between the two; among his criticisms is the fact that the late date of the surviving Irish texts means influences can enter directly from the French material, so shared motifs don't necessarily mean shared sources or Irish origins for those details.
He's also written about connections between Tristan narratives and the Naoise/Deirdre story, so looking at the love triangle motif in another Irish text. I haven't read this article yet, but it's here, if it's of interest (not sure if paywalled or not, sorry, I'm on institutional WiFi right now so it's bypassing all of that!)
Another article that looks valuable on this topic is Joseph Falaky Nagy, "Tristanic, Fenian, and lovers’ leaps" in Diasa díograise: aistí i gcuimhne ar Mháirtín Ó Briain (2009). Unfortunately, I can tell you nothing about this article because this book is not available to me. I have actually just requested that my library buy it because a couple of the other articles in there are relevant to me, but that means I can't advise on its contents yet!
Nagy has another article on the topic, 'The Celtic "Love Triangle" Revisited', in An XIV Comhdháil Idirnáisiúnta sa Léann Ceilteach, Maigh Nuad 2011: Imeachtaí (Dublin, 2015). Again, I have not read this one (although it looks like my library does actually have it), but it's also cited in Theuerkauf's article and sounds like it could be helpful.
I don't know if this answered your question at all, as I feel like the general vibe of what I just gave you was "ehh well they're definitely similar but the question is Why they're similar and that's more complicated" (ain't that always the way). But hopefully it might give you some sources to follow up on. I'm sorry I can't provide direct links to more of these -- the lack of open access and digitised scholarship in this field is a challenge.
*A point of terminology just to clarify in case anyone was confused: 13th-14th century may not sound "Early Modern" if you're used to thinking in purely historical terms, but linguistically, when it comes to Irish material this label tends to start from about 1300 (and obviously, being found in 17th century manuscripts positions this tale more solidly in the early modern period anyway).
