Would a native speaker say that?
Basing on FGB, I would translate this sentence as an gcreideann tú uaim? (as meaning do you believe in what I said?).
The two examples with object in nom.-acc. (?):
- Creidim sin (I belive that),
- Creid uaimse é (You can take it from me.)
suggest that creid + obj. means believe that the obj. is true.
So I would interpret an gcreideann tú mé? as do you believe I am [a] true [sentence]?, which makes not much sense…
EDIT: OK, there is also creid mise (go) in FGB, which I missed before, and which seems to confirm this sentence.
In (my dialect of) English, the verb "to believe " can also express incredulousness, as in "Can you believe she wore that hideous dress to the party?" There's no question of the existence of the hideous dress, or that she wore it. Does Irish do this too, or is there a different word?