Even with a newly built stone wall there inevitably occur voids in the mortar (too large areas), between facing & bond stones.
These voids must be filled with carefully chosen slim pieces of stone called hearting. This is literally the heart of a good wall.
Hearting stones, or cloch feidín, are typically made on site by breaking larger rocks into various size wedges. However on an old site like this, plenty such stone already exists. Poor quality stone walls can be identified when the builder simply pours gravel into the voids.
The tighter the hearting, the stronger the wall.
Hearting stones are much better if they are flat or angular, and should be placed individually. Make sure each course (row of stone) is completely hearted before beginning the next course.