You often find this expression but I doubt it is correct

Táthar ann a deir ...

It should be: Tá daoine ann a deir ... = Some people say ... / There are people who say

Táthar is impersonal. And impersonality cannot be "ann" only someone personally can.
And it seems to be distributive - not everyone says but only some of them say - which as well is incompatible with the autonomous form.
And, furthermore, the "imperson" is antecedent of a relative sentence which I doubt is possible at all: "... who says" but who is who who says?

It is probably due to a mix of táthar and tá daoine or deirtear and deir daoine.
Daoine is often used impersonally and replaces the autonomous form.
But the saorbhriathar cannot replace daoine vice versa in every situation.
in "tá daoine ann a deir" daoine is very concrete and personal not impersonal at all and so cannot be replaced by táthar.

What do you think?

I'm sure you right, Labhrás.