That "concrete septic tank" keeps showing up in posts. There was none.

A slab the size "of a coffee table" covered the remains of a number of small children. This has been reported in the Irish Times, and was mentioned earlier in this thread. This conjectorily was the location of a shaft burial vault.

Maps show the area of the burials as a septic or leach field in the days when the establishment was a workhouse. The operation had ceased when the buildings and grounds passed to the nuns.

Catherine Corless, the historian who broke the story (see Irish Times, June 7, 2004), concluded that many of the 796 children had been buried in an unofficial graveyard at the rear of the home. The grassy space has been tended for decades by local people, and a small grotto was put up in one corner. (Bold type is mine).

In long decades past, the area was a waste disposal area.