I believe what had happened (this was like 3-4 years ago that I watched the movie) was the child was left in some small space and she died there. Maybe she had hid there though...I really can't remember for sure. I know that when the new family went looking they found an old doll in the place the child had died. I wanna say that they had to break down a cement wall in order to find it.
It may not have been a castle. I think the title was something like, "An Irish ____". I know the name they used for the castle/house was an odd name that isn't typically used. I've tried looking under chateau with no sucess, though that is the name I've found that seems most right.