I did it. It took a year, but I really did it.
As of today, I have not only reached my 365-day streak but also fully completed all of the Irish language Tree. All Skills are now gold.
The next step is clearly to go back and review a lot of the Skills I haven't looked at in a while. (One year is easily enough time to forget which things I've already forgotten!) After that, I'll start looking around to see what other things I can find to keep progressing down this road.
For now, though, I am just going to give myself a moment to take in that I accomplished what I set out to do. It took about 20 pages of handwritten notes in a notepad as I went along, and more than a few nights of hurried scrambling when I realized that I hadn't done any lessons yet by 11:30 at night. I don't even want to think about the number of friends I've bored while muttering incoherently about the way verbs sometimes get discarded altogether since prepositions are perfectly capable of doing the same job instead.
I cannot thank enough all of the people along the way that have helped answer my questions when I've gotten stuck or confused on some bit of grammar or terminology. Knowing that there is an active community of people who are all invested in helping others learn this language has been a constant, buoying force along the way. Go raibh míle maith agaibh.
I feel that the Irish course, in its current state, is not set up to bring people up to the level of fluency in the language by itself. One of the biggest handicaps to this is the lack of any robust text-to-speech or speech-to-text software, and that is something that the creators of the course don't have any control over in any way. As it is, the entire course uses only one single native speaker for all spoken sound samples. (And for that matter, her spoken lines become noticeably less and less frequent the further down the Tree you go.) If I ever met her, I might be able to hold a simple conversation with her as long as she spoke slowly. Other than that, I doubt my ability to follow a different person's voice (or dialect or accent) if they were speaking at full conversational speed. Even then, the course does not provide any tools to help learners with pronunciation, so I would have trouble speaking the words that I do know correctly.
On the upside, the course DOES serve as an excellent starting place. On the one hand, I have become more and more familiar with online resources like Teanglann and Gramadach na Gaeilge which offer a level of detail about words and grammar that extend beyond the scope of what the Irish course is designed to offer. (There are probably several other important resources that I am neglecting to mention as well, but there are a multitude of threads already in the discussion board about online resources that cover the subject better than I can.) On the other hand, I am looking forward to browsing YouTube for Irish language programming with subtitles -- the next step in the process will be to expose myself to hearing new voices and learning the cadences of natural speech. While fluency might be out of my reach at the moment, progression with the language is not.
As for my future on Duolingo, I feel that it's long past time to learn Spanish so that's going to be my next project. However, I still plan to jump back in to do a few practice exercises in Irish along the way in order to try to maintain the knowledge that I've already worked so hard for by now.
And that was certainly a longer answer than your question was! I hope it was helpful.
No reaction to spending time on Castellano but my suggestion for Irish, watch https://tg4.ie/ga/clair/ros-na-run/ Ros na Rún on tg4 twice a week with captioning as Gaeilge, toggling to English from time to time when needed ... you'll get a wide variety of dialects and a really deepen your ability to speak. There are lots of other interesting shows on tg4 in Irish with subtitles in English or Irish too https://tg4.ie/ga/player/boxset/.
Breaking out my calculator, it looks like I averaged about 100 xp per day, but I know that number was VERY front-loaded and also kinda all over the place. With any of these courses, the initial Skills are always super easy to blow through and I spent a number of weeks earlier on deliberately knocking out something like 200 or so xp per day. However, that certainly slowed down a LOT when I got to the later exercises which required more care and thought. I probably spent at least a couple of months doing 20-30 xp per day. Then, the last couple of weeks I realized how close I was to finishing and really cranked it back up to around 150 per day specifically so that I could hit both benchmarks on the same day. (It was no accident that I only had 5 exercises left to do to finish the course on exactly that day.)