All the different cases of nouns, the random rules for initial mutations, needing to know the feminine and masculine form of everything, the differing dialects, all the different plurals, the subtleties of pronunciation and probably more that I'm forgetting...

This language is so hard to learn!! Goodness gracious!!! /rant

Keep at it is all I’ll say. Language learning is a marathon not a sprint. And everyone feels the pit of despair while learning!
How are you approaching learning atm?

I have the PDF of Teach yourself Irish which is my main source. The main thing I'm doing is just reading the exercises/answer keys over and over again and listening for pronunciation. I also read the chapters sometimes of course. Then I have Rossetta stone that I borrowed from the library which I'll just turn on one of the CD's and follow through the books that come with it. Mainly I just listen to the "conversation" sections over and over again. I also have an interlinear New Testament that I read sometimes, as well as an interlinear Irish poetry book "An Duanaire 1600-1900" (which I'll admit, I'm not overly impressed by much of the poetry). Also, I'll take certain bible verses and then find an Irish translation that I like and study the syntax/grammer of the sentence very closely until I completely understand it. Soon I'm going to get a copy of PUL's translation of the Bible (what he's done so far anyway) and so that'll be nice; I'm going to print it out and get it bound into a book. Also, PUL has an audio recording of the first 7 chapters of Gospel of Matthew, which I listen to a decent amount.

I'm a bit all over the place I realize. I'm just immersing myself in the language from every angle that I can. Someday I do want to go to Ireland for sure.

- All the different cases of nouns?
Only four in Munster Irish, but the vocative easy - for the 1st declension, memorize the inflection of 'fear'. (Also, it's not like you're going to go out and shout at fish 'a iasca, a chairde ionúine!'). The dative is trivial.

- the random rules for initial mutations,
There are quite a few rules, but if you take just one dialect into consideration, you just need to memorize a small table and that's it.
Unless you're trying to learn all the dialects at once…

- needing to know the feminine and masculine form of everything,
like in most European languages - 2 at least, that is, because German, Dutch (Standard Dutch) and all Slavic languages have 3. See, Irish is easy!

- the differing dialects,
I'm learning Munster Irish and I've noticed that the better I get at it, the better I get at the other dialects too. I think that's because I acquire a command of the language as a system. Say, Ulster Irish an+prep. causes lenition (except d,t). Since I know the Munster (Cork) variation, and this rule, I know how Ulster Irish works (in this regard).

- all the different plurals,
now, that is difficult. Fortunately even native speakers like, perhaps positively enjoy, using a variety of them. E.g. crann, m. tree. Plural: crainn/cranna/crainnte/crannaíocha.

- the subtleties of pronunciation
if you can distinguish broad and slender consonants long and short vowels, you're golden.

- and probably more that I'm forgetting...
conjugated (inflected) prepositions Oh, and the numbers too. And did I mention interrogatives?

- This language is so hard to learn!! Goodness gracious!!! /rant
Not as hard as Chinese for a native English speaker (taking only the language as such into consideration. The availability of resources, media etc. is an entirely different issue.)

This was supposed to be encouraging… Was it?

What definitely helps is listening to audio 100s of times until it sticks - which you do, so keep at it! I cut the audio from Teach Yourself Irish (1961) to remove the English and pauses. Perhaps you will find it useful: https://archive.org/details/TeachYourse ... Recordings

Also, say it out loud, repeat simultaneously (= shadowing) or after the recording. Also, flashcards.

I started with TYI because I am interested in the Munster dialect. But I found it dense and, obviously given its age, old-fashioned. I switched to Colloquial Irish 1 and I have made a lot more progress than with TYI. It follows the TEG structure which follows a international standard for learning languages. It sticks to practical language in groups (greetings, holidays, work, etc) and introduces grammar slowly. There is an annual exam that you can take and an online test to measure your progress. I am now onto Colloquial Irish 2. Colloquial Irish 1 + 2 focus on the Cois Fharraige dialect but it does address other dialects as well.

I am not suggesting that Colloquial Irish 1 is 'better' than other courses including TYI but I, personally, as a beginner, found it more accessible.