Hi,
I came across a similar problem when trying to hang up coat hangars on a wall in a 100 year old house. As I started drilling little stones started coming out of the wall and it appeared that the wall plaster (or what was underneath it) was a mix of sand/cement/pebbles. I think it is just what was available back when the house was built.
What I did, and it worked really well, was to cut small squares of wood and nailed these onto the wall, covering the holes I had made. I found that the nails held the wood very securely to the wall. All I did then was drill holes in the wood and secured the coat hangars to the wood.
We have lots of coats on the coat hangars, plus I would say that it could take a lot more weight.
Padhraic