A story, that makes you doubt a lot of things happening on screen. One may think that's not, good because often you need something believable, this is not the case here, where it is the whole point of the show, or at least its first half.
Lorna Brady suffered a lot as a victim of a "laundry", where she was abused for cheap labor and where her child was taken away. Hers and of hundreds of women like her. What's worse almost no one believes the survivors that the church, where they were living, was a "laundry", that it could be this "evil".
This is still not enough, though, because Lorna has suffered so much, that it broke her mentally, she sleepwalks doing crazy stunts, and people around her keep telling her that she would not be a fit mother anyway. She is utterly devasted and withdrawn. Every moment of the show makes you wonder if she is just insane, even when someone tries to believe her for a moment.
I am not sure how they managed this, but the way actors... Acted, I guess, it felt different from what I am used, too. It never felt like they were acting. It felt so real. My guess would be that it was because of little imperfections: in wardrobe, in movements, in words. It was captivating.
But that's not all. The story is great, too. The way the mysteries unfold is almost like a flower blooming. And it all makes sense, the timings are almost perfect (with the "almost" making everything more believable). Even the ending, while it does provide some satisfaction (and make a heart broken in a good way), it does not magically resolve everything, it's not a "happy end", but it still feels really good.
There is one "complaint" from my side, though:
Aoife ends up dying after getting out of the wall and reaching the attic, right? Presumably because she was not able to get out of it. But her body placement does not make sense, because if she even tried to get out she would have found the trap door. And the trap door was shown in the last episode: it was the one that opens down it looked relatively flimsy. Even if stomping it would not have broken it, dropping something really heavy multiple times, probably would.