Most weren’t orphans. That was a home women were sent to if they were pregnant and unmarried. They should have been with their relatives. But the Roman Church made that near impossible. Almost as charming as their reason for dumping the dead babies in the septic tank. Children of unmarried parents could not be baptized, and therefore could not be buried in a churchground.
Blaming Catholics today for this is unfair,, unless they want to go back to the old ways. I doubt the modern church would do this. But the old ones were some harsh mean SOBs.
With no health service and deep poverty, the number of children working class parents had to watch die was horrendous. Infant mortality soared. In 1926 in Ireland 120 of every 1,000 babies under the age of one died compared to six of every 1,000 today.36 As late as 1949 over 50 of every 1,000 babies died before the age of one. One child in 16 born in 1949 did not live to see her or his fifth birthday. Diarrhoea and enteritis were the biggest killers of babies. Tuberculosis and other preventable and treatable diseases swept through the slums, killing older children. All these children died of poverty.37
http://pubs.socialistreviewindex.org.uk/isj91/horgan.htm