Translation:Books are written about us every year.
In modern Irish, it really represents an unidentified agent in the active voice rather than a passive voice (“One writes books” rather than “Books are written”), although it’s often translated into the passive voice in English, and was historically a passive voice. Remember, Irish is still in beta, so be sure to provide feedback wherever you deem it warranted.
It's not just that. The report button allows you to give lots of other kinds of feedback too.
Still, the autonomous form of the verb isn't really a passive, though it is used, as scilling mentioned, for impersonal statements, which is what the passive is usually used for in English. and other languages. You should probably suggest that some mention of it be added to "Verbs: Present 2".
The autonomous form is really easy to spot. In the present tense, it's always indicated by the ending -tar.
You might find this description of the regular verb conjugations useful: http://www.nualeargais.ie/foghlaim/regular.php
You were marked wrong because "One writes books" is not an accurate translation of this exercise. Scilling's comment with regard to this particular exercise was misleading.
"They write books about us", where "they" is a nonspecific/generic "they", rather than an identifiable group of writers, would work, for people who think that "voice" is more important than introducing ambiguity, and who don't think the fact that the saorbhriathar specifically avoids specifying the actor is important.
I'm not entirely sure what your definition of "the indeterminate" is, but are you seriously claiming that "one writes books about us" and "they write books about us" are synonymous?
In my experience, American English allows the use of "one" as a replacement for the "indeterminate you", but not for the "indeterminate they" (and that's making an assumption about your usage of the term "indeterminate").
So could Scríobhtar leabhair fúinn gach bliain. be correctly translated as "There are books written about us every year?"
And on a tangent: I had thought that verbal adjectives in Irish correlated pretty strongly with past participles in English; is that not an accurate characterization?
Answering tangent and question in general. They do in some places, but not in others. Like, you can say Tá sé imithe, which means "he is gone", so it could perhaps be used. Especially without an ag with it.
However, "books are written" sounds odd to my English ears, and I personally think "There are books written about us every year" sounds as a better translation. So you could probably do use Scríobhtar leabhair fúinn gach bliain.
It's when you start getting them into perfect structures that they get iffy. For example, Tá sé déanta agam does not generally mean "I have done it." Instead, it's more interpreted as a stative perfect, "I have it done." Most grammars will tell you they do, however, but I feel this is a form of projection of English onto Irish.
Really, the whole concept of the perfect and VA in Irish is a pain. I suggest finding and reading Diarmuid Ó Sé's The Perfect in Modern Irish (Éiru, 1992).