Several A&E departments have been hit with a shortage of doctors as they face into the annual winter trolley crisis.
A number of freelance doctors, who are crucial to working shifts left vacant by a lack of permanent medics, did not turn up for work yesterday in wildcat work stoppages.
The decision follows the start of a pay cut for these locum doctors that came into force last Friday.
Senior consultants fear that some A&Es could "cease to function" if the shortage escalates.
The warning was sounded last night by the Irish Association for Emergency Medicine, representing A&E consultants.
It comes as figures showed 7,781 patients who should have been given a hospital bed were on trolleys for hours last month, the worst August for overcrowding in a decade.
A spokesman for emergency consultants said the significant pay cut for locum doctors has meant some did not turn up for their shifts yesterday.
"It has also made the filling of short-term vacancies less timely and more likely to be unsuccessful."
He said the association "is acutely aware that in some hospitals the majority of medical staff are employed through locum agencies and in these hospitals the emergency department will cease to function in the absence of doctors".
The emergency task force, attended by Health Minister Simon Harris, met yesterday to discuss plans for winter overcrowding.
Mr Harris said a national hospital bed review is nearly complete. This will provide information on which hospitals need beds, and how many, over the coming months.
However, the 2016/2017 €40m winter initiative plan launched last autumn missed several targets.
It included a proposal to look at putting up prefabs in some hospital grounds where some patients could be appropriately care for.
This led Independent TD Michael Lowry to declare that a "patient hotel" that would accommodate people who were attending South Tipperary General Hospital would be built.
Asked what progress has been made, a spokesman for the hospital said it is not possible to provide an update at this stage.
"Funding approval for the modular build is still awaited," he said.
The Inmo's figures yesterday show that South Tipperary General Hospital suffered the third-highest level of overcrowding last month.
Some 489 were on trolleys compared to 82 during the same month a decade ago.
Plans for a similar prefab building in Mayo General Hospital have also made slow progress.
A spokeswoman said: "Mayo University Hospital is currently seeking to progress to a mini competition locally. At this stage in the process, we are not in a position to confirm timeframes or costs."
Other missed targets included those for staff getting the flu vaccine and, crucially, waiting times for patients on trolleys needing a bed. Hospitals outside Dublin suffered the worst conditions last month, including University Hospital Limerick, which has the newest A&E in the country.
The nurses' union called for senior emergency consultants to be present in A&Es over seven days. It also wants nurse managers to be given full powers to recruit additional staff. Hospital managers should also be on site on a 24/7 basis, it added.