@Peter Laban
The starting point of the trail which came to this conclusion appears to be the public documentation of the deaths - Mrs Corless was researching the Home as an Institution, and started talking to the residents of the Estate built on the site. From that she learned of an "angels" graveyard, supposedly of babies dead before baptism, and went to the Galway Registrars, where she learned of nearly 800 deaths, far more than could regularly occupy the space as normal burials. That then lead to the discovery of the 1970s exposure.