#i am emphatically not a linguist and cry if i have to do textual dating based on linguistic features btw

[finnlongman]

If one were to want to read the Irish myths in the original language, would it be better to learn old or middle irish? how much difference is there between them? a lot of resources/books I've found focus on Old Irish, or at least that's what the titles say. But when I've looked at the original texts for a lot of the tales, i cant actually tell if what I'm looking at is old or middle irish.

Neither, do Early Modern Irish and join me in hell, lmao.

Realistically due to manuscript survival, the majority of longer narrative texts are in Middle Irish or later, so if your interest is "reading stories", that's what you're going to end up dealing with. Very, very few people would just sit down and read a medieval Irish text the way you might read a passage of modern Irish, though this is partly because many of them are in dreadful editions that require a lot of work just to use, and in turn this is often because of illegible or damaged manuscripts. A more realistic goal would probably be trying to get to the point of being able to translate medieval texts, or to compare other translations with the original.

But most learning resources for medieval Irish are Old Irish. That ... doesn't really matter? Like, you start with Old Irish because it's the most complex grammar that's the most different to, e.g. Modern Irish. And then you just sorta relax your grammar and lose your infixed pronouns and gain independent pronouns etc and you wind up with Middle Irish without having to learn a separate thing. We had exams on trying to tell the difference when I was doing my MA and it can be Challenging, especially since there's a lot less definitively Old Irish than people originally thought, and because things are often preserved in later manuscripts with updated spellings.

Plus, many of those horrible editions that I mentioned were done by editors who decided to reconstruct the spelling according to what they thought it should be, or standardised all the verbs, or otherwise emended the text in such a way that a lot of its definitive dating features are now invisible. So if you're looking at an edited text, it can be incredibly challenging to gauge the language and thus the likely date.

Anyway, the textbooks like to show you nice proper grammatically correct Old Irish with all its infixed pronouns squarely in place and behaving themselves, and then no text you look at will ever look like that, at all. It's great fun. There just aren't textbooks and resources for learning Middle Irish specifically (probably because anyone working on Middle Irish is going at some point to need to deal with Old Irish features, and also because of the field's historical trend towards focusing only on the oldest versions of things they could find). But you will find resources for Old Irish that incorporate texts with Middle Irish features.

You can also go the other way, which is to learn Modern Irish, which is much easier due to the wider availability of resources and teachers, and then just kinda work backwards. I did not do this so I cannot advise on the best approach there. I will say that I am finding my modern Irish more useful for most of the Early Modern stuff I do than my Old Irish, but that is also because I have substantially more modern Irish than I have Old Irish at this point, and the Old Irish at least aids me in guessing how to approach particularly heinous verbs.

(It is miles easier to eyeball an untranslated 15th century text and have a vague sense of what it means than it would be to do the same with a tenth century text, especially with how rusty I am on Middle Irish at this point. So if I am posting excerpts of a text and noting that I'm just doing the translation on the fly it is going to be an early modern text 99% of the time.)

Rough dates, fyi: 600-900 Old Irish, 900-1200 Middle Irish, 1200- Early Modern Irish. Realistically you have Middle Irish texts being written in the fourteenth, fifteenth centuries and of course things also get copied into later manuscripts and preserve older language, so these are only very broad approximations. TBC 1's earliest MS is ~1106, the Book of Leinster is ~1160, so firmly towards the end of the Middle Irish period, but they still have bits of Old Irish in there, esp. R1. Stowe is 15th century in a 17th century manuscript with modernised spelling but often it's Middle Irish in grammar. It's a whole mess. Good luck lol

[Apologies that this is not the most coherent or detailed answer I've ever written. I am very tired right now]