Could the nuns have somehow been in ignorance about what they thought was a burial vault? And does ignorant mistreatment of dead bodies, if that is what happened, need to be treated as a huge scandal once it is discovered? Doesn’t it suffice that they bring in someone to bless the site and that takes care of it? I doubt any of the souls of the dead cared where their bodies were....
There are vertical "shaft graves," also known as "pit graves," also known as "vertical pits," all over Ireland and in fact all over Europe. Each pit is a deep hole lined with bricks, then inside lined with plaster.
If refuse (including human waste) is put in it, it is known as a "waste pit."
If water starts seeping through while it's being dug, it is finished off as a "well."
If it is blessed and used for the interment of the mortal remains of a person or persons who have died, it is consecrated as a "grave."
The point being that these things are, structurally speaking, practically identical, distinguished only by use. And there is no way on God's Green Earth that nuns would have "thrown" or "dumped" bodies into a septic tank. They were interred in consecrated ground.
The ignorance --- and malice--- evinced in these "septic tank" articles is just astounding.