The Irish Nurses' Organisation (INO) has objected to the HSE after 39 newly qualified nurses in Sligo were advised to register as agency nurses because there are no contracts available in Sligo General Hospital.
The nurses' union has said this is in breach of a pledge given by the HSE last year that all newly qualified nurses would be offered a minimum six-month contract.
Noel Treanor, industrial relations officer with the INO in the north west, said the reason being given was that Sligo General Hospital had exceeded its allocation of nursing posts.
"They are needed in the hospital but now these newly qualified nurses have been told that they have to join an agency and they will then be employed on an 'as-and-when' basis," Mr Treanor said.
He added that the HSE's human resources department had made the pledge to give six-month contracts in an effort to ensure more nurses were kept in the Irish health service.
Mr Treanor said telling the nurses to register with agencies was short-sighted and would cost the HSE more in the long term.
"Management has indicated it is more expensive to employ people as agency nurses than to give them contracts so, because of restrictions imposed by the Department of Health and the Department of Finance, they will have to spend more money," he added.
In response to queries from The Irish Times, the HSE issued a statement saying that Sligo General Hospital would be recruiting additional nursing staff "through the normal recruitment process as and when required".
Mr Treanor said newly qualified nurses in Co Donegal had been given contracts in Letterkenny General Hospital.
Of the new graduates in Sligo, who studied at St Angela's College in the town, Mr Treanor said one had been offered two and a half days work in Sligo General Hospital.