Talks + Acts Very Child-like

Well, she can't really do anything about her voice. I agree that it might be considered annoying, if it was an affectation, but it's not. How do think she acts child-like? I've never noticed this. She is quite petite, so perhaps her stature in relation to taller actors, might make her appear "child-like". Looking up, talking to others. She also has quite large eyes, which might add to the 'doll effect'. None of these things are really something you can blame her for. In 'Southcliffe', her character completely breaks down. The crying, and helplessness is behavior we usually attribute children. That might also add to you impression.

Sometimes we just find a person/actor, annoying. I know the feeling. Just not with Shirley Henderson.

Unlike you, I find her characters, often to be quite powerful and strong. She was just incredible in 'Southcliffe'. It's been a while since if felt as uncomfortable, as when I watched the "prostitute job interview" scene. I think she was just great, and so was the series.

When she is talking in her Scottish accent, she doesn't sound like that (see Hamish McBeth). I think it is just dependent on the role she is playing i.e. Moaning Myrtle in Harry Potter or in the Agatha Christie Miss Marple 'Murder is Easy'.
She is also only 5 feet tall & slight build so she will look diminutive next to average height or above actors.
I think she is a brilliant actor.

It's called ACTING, you miserable, whiny, sodding idiot. She's won 10 times more awards than the amount of lays you've had or will in this lifetime, too.

Whoa. Calm down there, pal. The original poster was just stating that s/he finds Shirley annoying and baby-ish in her mannerisms. They didn't state that she couldn't act.

Yes, freaking annoying!