Nurses and midwives are to begin a strike ballot today.
The INMO says its because of the number of unfilled jobs in nursing and midwifery, which mean existing staff are left doing unpaid duties and working on their days off.
It says even 'essential' posts are taking up to 12 months to fill.
General Secretary of the INMO Phil Ní Sheaghdha says the HSE claims there are no staff shortages.
"We knew there was a great deal of reliance on goodwill - we didn't know the extent of it until we spoke to our members, and we did a very quick straw poll survey.
"Every single person has been contacted to come in on their days off because of shortages, at national level the HSE are saying there are no shortages.
"The bottom line is it's going to get worse."
