I probably can’t offer much more than the dictionary definition which is that a nursemaid was someone employed to look after another person’s children.
If you read reports on workhouses in the 1800s, like the one below, you will find descriptions of the medical facilities. Usually there was a doctor and a very small number of nurses. Barely sufficient to give proper care to all the patients and in report after report you will find recommendations to employ more salaried nurses. In the meantime, what the workhouses did was use a few of their pauper inmates as “nursing assistants” you might call them. They could be responsible for non medical tasks like cleaning patients, serving and clearing meals, tidying the wards, and so on. All workhouses had women with babies and young children and so presumably some duties may also have been around their welfare.
A bit about Mullingar Workhouse here, plus the report of an inspection in the 1890s.