The INMO is calling on the Government to implement a major cost-saving measure agreed as part of the nurses pay deal.
A staffing framework to determine what staff are needed based on the number and condition of patients was identified as a way to save over €70m a year.
It was piloted in three Irish hospitals and proved to practically eliminate the need for agency staff.
Today's Irish Times claims the cost of the nurses pay deal had been grossly underestimated by Government.
However, INMO General Secretary Phil Ni Sheaghdha, said the potential savings were overlooked.
Ms Ni Sheaghdha said: "The savings, as I read the Irish Times article today, are not included at all and we know that conservatively there are savings in the region of over €70m a year to be achieved if the framework in our staffing is implemented immediately.
"That's the key, they need to get on with it."
The INMO has also warned that patients are facing serious risks, delays and service cuts due to a HSE recruitment ban.
It has sent an initial list of vacant nursing and midwifery positions to the HSE, which are not being filled due to an embargo on hiring.
They claimed that services across the country have been compromised, giving the example of Co. Wexford where, they say, there will be no provision for immunisation from September.
The organisation described the list as “the tip of the iceberg”, as it only contains unfilled posts which union members had raised with the organisation.
It claimed that, in practice, some services are replacing full-time staff with temporary agency workers due, in part, to the recruitment ban but also due to the recruitment process "often taking up to six months".
Ms Ní Sheaghdha, said: “The recruitment ban in nursing and midwifery has got to go.
"Before the ban, we had understaffing and overcrowding. Refusing to fill vacant posts is adding fuel to the flames."