First off, I love YouTube, there are plenty of great resources on there, and plenty of great players who post/have posted regularly. This is not a warning against YouTube or against tutorial videos in general.
I have noticed, though, that there are a more than a few people on the site offering “tutorials” and teaching videos that leave a lot to be desired. Their playing is often not particularly good, but some will slap a bunch of reverb and/or backing tracks on it to buff up the sound. In fact, these videos are often somewhat slickly put together, with pop-up tabs and good editing. Many specialize in video game music, movie themes, and other non-trad music. In and of itself that’s not a bad thing (far from it, it’s fun to hear other music played on the whistle!). The issue is that they also try to teach Irish tunes, which inevitably goes horribly. Not only are they not strong players, they don’t have a grasp of trad as a style, and so they end up teaching a bastardization of whatever tune they’re attempting. Worse yet, some will make pronouncements about Irish music that are misleading, ill-informed, or just flat-out wrong.
I don’t necessarily want to “name and shame,” but one video came to my attention recently that was particularly egregious. The poster was “teaching” the Rocky Road to Dublin, a popular tune that is well-suited to beginners. The player in question not only plays the wrong notes and rhythms, they wrote in the description that adding and taking away notes is perfectly ordinary. Presumably they took the idea of “variations” to mean “anything goes!” Their other videos featuring Irish tunes are similarly poor, and they don’t demonstrate a high level of playing, in Irish music or otherwise.
There are a variety of ability levels on this forum, including a lot of beginners. If you’re looking for tutorial materials for Irish trad or tin whistle playing in general, please be critical about what you turn to. Listen to the “teacher” playing, and compare them to well-known whistle players. They don’t have to all be at the level of Mary Bergin, but it’ll help determine if this person is really in a position to be teaching. If they’re teaching a tune, listen to other versions of that tune to make sure that their version lines up. Also, if you’re looking to specifically learn a style like Irish trad, be sure that the person actually plays in that style and has a good grasp of the unique techniques, repertoire, and sound of that genre. Otherwise, you’ll be learning things that you’ll eventually have to un-learn. If you’re unsure about a certain resource, ask about it on this forum, TheSession.org, or at your local session. There are tons of great resources available (Ryan Dunn’s videos come to mind, despite a too-liberal use of reverb IMO), and more experienced players will be happy to point you in the right direction.
And if you’re thinking of making a tutorial video yourself, PLEASE be honest with yourself about your own limitations. It’s great to want to help beginners out, but you have to be sure that you’ll actually be helping, not impeding their progress.