The INMO has said the HSE's recruitment plan for new nurses has failed and called on the Department of Health to intervene.
The Irish Nurses and Midwives Organisation said there has been an "alarming drop" in the number of nurses and midwives within the health sector.
It said it has met senior Health Service Executive managers recently who have confirmed that they are being refused permission to hire nurses because of budget constraints.
The organisation said this contradicts with the HSE's stated position that there is no hiring moratorium.
It also said that unfilled nursing positions continue to increase and there are now 270 less staff nurses in the public health system than there were in 2015.
Speaking on RTÉ's Morning Ireland, INMO Director of Industrial Relations Phil Ní Sheaghdha said the HSE's recruitment plan was never going to succeed in the current market.
She said the UK is aggressively recruiting nurses with representatives coming to Ireland to recruit nurses six months before students graduate.
Ms Ní Sheaghdha said more than €22 million is being spent on agency nurses and this could translate into the creation of 646 posts for nurses if recruitment went ahead.
She said while the HSE is saying recruitment of nurses is allowed, this is not the case on the ground.
"We met two hospital groups this week, the Saolta group and the Mid West group [which provides acute hospital services in Limerick, Clare and North Tipperary] all have vacancies. They have been authorised not to employ and offer permanent posts".
She said the Saolta group – which is made up of hospitals at Letterkenny, Mayo, Merlin Park, Sligo, Portiuncula, Roscommon and University Hospital Galway - has said it has been told to only authorise three month contracts.
"No nurse will remain in Ireland for a three month contract. You just can't plan, particularly when the UK will offer you a permanent post, a 37-hour week, more money and postgraduate education. We're just not at the races," she said.