Making of irregular plaster

Let's have a look at the complete material node.
As you can see, the final material is composed of the base plaster material and another material that will manage the edge irregularities, mixed together with a noisy dirt map.

As first step, I started with a simple plaster texture (I chose one with some tiny cracks on it, but you may use whatever you want of course), and then smoothed it out a bit by mixing it with a plain color.

Adding some kind of noisy dirt to the inner edges, to gain some lighter, irregular effect.

And to the outer edges as well. This is more important since having lighter outer edges will enhance the irregular effect when we will merge the final material.

Now comes the material that does the trick.. and it's actually the simplest thing!

Using the rounded edges parameter, we will simulate some irregularities.

Well of course we don't want the irregular edges to get all over the walls, so we'll use a contrasted, noisy dirt map to make them happen in some spots only.

Here is a shot of the mask for rounded edges in action.