Ireland like many in many places in Europe had a terrible fear of Faeries or bad spirits stealing new born babies from cribs at night time and leaving a changeling in its place. There were two reasons for theft of babies, first was to bring new blood to prevent too much inbreeding in the faerie population. The second was probably the more common reason was that faeries had great difficulty with childbirth and faerie babies often died or were deformed, these were either replacements for the faeries or used as servants in the otherworld.
Another way was to put the child into boiling water with the same results expected as the fire, with the faerie escaping up the chimney. Some infants were left in a manure pit overnight, if they survived the morning they were not faeries! One of the ones that I thought was the cruelest and was common in Ireland was foxglove poisoning which left the infant with severe stomach pains and took some for it to die. Leaving the worst till last, what I thought was the cruelest thing that could happen to an infant was a red hot poker up the anus, similar to the fire if the child was a faerie its insides would boil on the inside and the faerie spirit would disappear up the chimney, if not a faerie the child was spared.
It is obviously a form of Eugenics killing of a sickly or handicapped child that would take up a lot of time of the family, this practice was more common in the rural regions, were where everyone was needed to work the farm. There are other cases of older people and animals being taken by the faeries, these will follow again.
Credit for some of this information comes from a colleague Sean, thanks.