It was a converted septic tank, i.e. no longer in use as such. As a mass grave is was merely an underground cement vault.
It was a used septic tank left where it was, covered with a cracked cement top, and found by some school kids who were playing in the area:
Corless also said in 1995 several boys had found the mass grave, which lay beneath a cracked piece of concrete.
"The boys told me it had been filled to the brim with human skulls and bones. They said even to this day they still have nightmares of finding the bodies." (https://uk.news.yahoo.com/galway-horror-800-dead-babies-found-septic-tank-173247464.html#4F5aKPJ