I'm rather surprised to see that so many people here don't think they need to vet the books their children are getting from the library. I considered it an absolute obligation to know exactly what my children were reading at least into their first year or two in high school. (And even now, since we all seem to get a great deal of pleasure from discussing the books we read). I get the impression from many here that it is common in Ireland to just turn one's children loose in the library, assuming that the books are all good because the government says so. There are lots of reasons to keep an eye on books. Excessive sexuality might be one, but let's take a look at the particular book under discussion.
The book (based on the page that was posted) looks like a bad book. It's poorly written for one thing, and this alone should be a reason to avoid it. It also takes a very complex subject and vastly oversimplifies it, which means that what was written isn't really true. Well run libraries are nonetheless full of crap books and unless the parents have an overly compelling reason to have to delegate the entire education of their children to the State (say, for example, if parents were serving a long stretch in a Turkish prison) they should know what books their children are reading and they should also be discussing them. I think I can begin to see now why so many people are putting such a stress on the library doing this for them.
In the small section of the book that was posted, I don't see any mention of transexuals. I have not read the book myself (but then, I don't think anyone else here has either) but I am not seeing how a book, even this rotten book, might lead a child to rapid onset gender dysphoria. The author is apparently transgender, but so what? And this leads to a second point. It's one thing to say that the book has explicit sexual details that might not be good for 12 year olds, who are not yet equipped to understand what it all means, and another to suggest that the book might make straight kids gay or any kid into a transexual. I'm afraid that I'm not seeing the cause and effect here. I really don't think that a child who happened for some reason to turn to page 147 or whatever and read that page is going to become instantly "sexualized". The concrete thing I am hearing is that transexuals are somehow being "normalized" and that this in turn is making children want to become transexuals.
Transexual adults have been a "thing" for something like 80 years. But they didn't become a panic until recently. I don't really know when minors started being treated for gender dysphoria, nor how many are being treated. But I don't think you know either. I would find it astonishing if it were an easy thing for children, especially in Ireland, to easily transition upon demand, or that any have demanded to transition based on the books they are reading at the age of 12.
It's not homophobic to have concerns about the contents of a book, unless one starts saying that children who are exposed to homosexual or transexual characters in books or discussions of the same are being "groomed" to become gay or transexual themselves. In that case, yes it is homophobic. Sorry.
I'll end by saying that I find your distinction between "accepting" and""normalizing" to be a bit odd. What's the difference? It looks like you are saying that children should be taught to accept that transexuals are freaks (and treat them nicely) but should not get the idea that they are not actually freaks.