As you can see, the damage is rather extensive, about 3' across and a 1.5' high.
I think that that's plaster underneath the paint, but I'm unsure what's underneath plaster. It does not appear to be concrete, as it breaks up rather easily into sandy bits.
Can I just break off the loose plaster and apply fresh plaster directly? Is there a bonding material I should use before I use plaster?
Please keep in mind I'm completely new to home repair; heck, I've never owned a house.