Shouldn’t the singular form of the noun go with this number, viz deichniúr tuismitheoir rather than deichniúr tuismitheoirí?
EDIT: As was helpfully pointed out to me, it’s not the singular form that should be used here, but the genitive plural form.
Is that just for the first ten, and only when refering to people?
It applies to all of the personal numbers (2 through 10 inclusive, and 12). In the case of 2, the governed noun is lenited.
Yeah, this is wrong, right? Shouldnt it be tuismitheoir?
No — the genitive plural is used, which for tuismitheoir is tuismitheoirí.
That’s how zú used to be spelled. (Back in the day, sú was considered slang for gairdín ainmhithe.)