Having read the book before this movie came out, I do feel a little biased. It seems almost impossible to make a movie better than the original book.
The biggest cons for me:
I felt that the casting was off, and the acting was extremely flat. The movie did deviate from the original story a bit, which is a given, but the addition of the whole parental death/self blame plot line was weird and clunky. Maybe I'm wrong, but I definitely don't remember that being in the book, and it just felt like space filler to try to give more unnecessary depth to a character.
The pros:
The imagery is great. Very atmospheric, without obnoxious jump scares every ten seconds. Overall it stuck to the original storyline very closely. Although the acting was flat, it wasn't horrendous. I did generally enjoy it. It's worth watching if you like Irish lore, and aren't looking for a hard hitting horror flick. I'd watch it again.