Hey! I was wondering, do you speak any old Irish, and if so, how did you learn it? Im interested in learning!!
I can read and, to a certain extent, write in Old Irish. (Most people don't speak it, but we are taught how to pronounce it...which was actually an impediment to me learning Modern Irish, because I'd had the older stuff first.)
There are a couple of options! The easiest, if you have the time + money, is always to take courses, which is how I did it. As part of my MA, I was required to take classes in both Old Irish and Welsh, and I've continued taking courses in my PhD. Now, if, for some reason, you don't have the ability to casually drop a few thousand Euros on a MA degree, there are a couple of sources I can point you to. If you do this, I can also try to walk you through it to the best of my ability, and I can assign you homework.
First off: Quin's Old Irish Workbook and Strachan's Paradigms. With most copies of Strachan's paradigms, like the one you see on Archive.org, it's kind of neat because people add notes to it over time, so it's also deeply personalized to the Celticist, like a family bible.
I'll tell you immediately: the current editions of both of these are quite frail. They fall apart quite easily, since they're softcover. (I eventually had mine custom-bound together.)
Stifter - Sengoidelic: Old Irish For Beginners. It's VERY in-depth, slightly intimidating, but there are lovely cartoons of sheep to make it slightly less scary.
Ranke de Vries - A Student's Companion to Old Irish Grammar
Antony Green - Old Irish verbs and vocabulary
Rudolf Thurneysen (trans. D.A. Binchy): A Grammar of Old Irish. I'm going to tell you now: This book is DENSE. It is technical. It is OLD. It is EXPENSIVE (especially if you want a good copy, since the one I got was so thick that it was actively falling out of its hardcover binding; I ended up buying an older copy.) BUT...it will help your Old Irish. It's still considered to be the definitive book on Old Irish, even though it's eighty years old at this point. So: I recommend it, but I'd recommend it after some of the others. De Vries is probably the most beginner friendly.
Now, after all that, you will need places to practice your Old Irish.
I highly recommend anything from DIAS' Medieval and Modern Irish series -- they tend to have very good dictionaries, so it isn't like you're being tossed into the deep end. Most of these are technically later than the Old Irish period, but they ought to give you a taste for the basics. (The problem with Old Irish is that Middle Irish was basically creeping in even from the time of some of our earliest texts.) One of these, which you can get online, is Compert Con Culainn and Other Stories, ed. Van Hamel.
Also: Thurneysen's Scéla Mucce Meic Dathó.
Ernst Windisch: Irische Texte mit Wörterbuch
If you can get ahold of it, Vernam Hull's edition of Longes mac nUislenn is also excellent.
When you're reading these texts, I recommend you to make a note of the case, number, and gender (nouns) or the classification, tense, number, whether it's adjunct or conjunct, and whether it's deuterotonic or prototonic, etc. (verbs). Mutations, including invisible mutations (nouns and prepositions). Every bit of information you can parse out, so you can instantly recognize words as you come across them and what they are doing in a sentence.
I'm sure you've heard this before, but Old Irish is notoriously difficult as a language. Having Modern Irish helps (particularly re: vocabulary, where you'll see a lot of familiar words), but the language has also radically changed. The verb in particular has totally changed the way that it's formed, going from a predominately synthetic language to a predominately analytical language (Munster Irish is closest to Old Irish in terms of verb formation, though each dialect preserves little bits of it.) Old Irish is significantly richer as far as vocabulary, and it had many more declensions (stems), each of which has its own way of doing things. You had more tenses. You had a neuter gender (though that was already going by the wayside). The definite article was totally dependent on the case, number, and gender of the noun (preserved these days in 'an' and 'na', but it used to be much more comprehensive). You had a full dative and accusative case (no ablative, thank God.) You had infixed particles, and those were divided into three classes depending on the classification of the verb. And adding to all that is that the language was already changing, so the Old Irish that you will be learning in the textbooks is not 100% what will be reflected in the editions. I've been doing this for six years and I still need to use a dictionary + paradigms. (Keeping in mind, though, that my Old Irish has also actively decayed in the States.)
The point isn't "don't do it" -- I'm not in the business of scaring people off. BUT I'm saying that it's okay if it takes a while, or if it seems overwhelming or even impossible. (It's also okay if it comes naturally; some people ARE naturally good at it.) The best piece of advice I have EVER gotten about Old Irish was what an older gentleman told me my first day of my MA program, which is: "Old Irish becomes a lot easier when you remember you'll be learning it your entire life." Take your time, enjoy it, let it sink in, play with it. Pursue the texts you want to read, read the texts you already enjoyed in English in Old Irish, even if it's a couple of lines at a time. ENJOY the process, and don't feel pressured to learn everything at once.
You have a lifetime to learn it.