I would not skim over that.
That looks like lime plaster and cement plaster patches.
And I bet the lime plaster has blown (just hanging onto the wall - sounds hollow when you tap it).
In my house where a skim was put on that, eventually the differential movement of the lime plaster with the cement skim means that new cracks form.
Also when I drill into it the (for say a picture) the skim peels away around the drill hole and the loose lime plaster makes a massive hole.
Take it back to brick (use a SDS drill with chisel) and re-plaster with insulated plaster on external wall (what I normally do) or board over the lot.