Minister for Health Simon Harris has said urgent procedures are being carried out in hospitals, despite the significant increase in patients waiting for a hospital bed.
The INMO has said that 541 patients are on trolleys in emergency departments and on wards waiting admission to a hospital bed.
The worst hit hospitals are University Hospital Limerick with 44; Letterkenny University Hospital with 36, while there are also 36 patients waiting for a bed at Cork University Hospital.
The INMO said 392 patients were waiting in emergency departments and 149 were on wards.
Mr Harris said a surge in the number of patients on trolleys in the Health Service was to be expected.
"We've seen a significant increase in the numbers in the health service. We said a surge would come and it has ... the flu has now arrived," he said.
Last week, the HSE warned that the virulent H1N1 flu strain – also known as swine flu – is in Ireland and could become a potentially serious health issue.
The Health Service Executive’s TrolleyGar figures show there were 393 patients waiting for admission to a hospital bed this morning, an 8% decrease on the same day last year.
The HSE figures only cover emergency departments.
The Irish Hospital Consultants Association said the large number of patients on trolleys has led to elective surgeries this month being deferred in most hospitals for part of, or all of January.
Referring to a report in today's Irish Times, Mr Harris said: "I'm reading about urgent cancer cases being cancelled, and that it not the case.
"Not as many electives are scheduled in early January. That always happens."