The HSE has confirmed that over 700,000 planned procedures for patients were cancelled since 2023 to the middle of this year.
It said that the term cancellation does not mean that the appointment did not go ahead.
The HSE said appointments and procedures are often rescheduled within a short period of the original appointment.
The HSE said that between January and June this year, 148,179 planned appointments were cancelled.
Figures show that in recent years, the number of cancelled patient operations has been rising.
For the first three quarters of this year, over 220,000 operations were cancelled, according to the figures published by the Irish Medical Times newspaper for its Patient Solutions conference today.
That compared with 286,545 cancellations last year; 267,426 in 2023 and 194,000 in 2022.
This would bring the total to almost one million cancellations in almost four years.
Stephen McMahon, Irish Patients Association, described the numbers as shocking.
He added that patients can be taken off the public lists without their knowledge.
Mr McMahon said there are a whole load of processes to validate the waiting lists in the system.
He said the danger is that if the system deletes people from the waiting list, without their consent and knowledge, this is a violation of their patients' rights.
The Irish Hospital Consultants Association said that relying on the cancellation of scheduled care to manage daily pressures is not sustainable.
It added that the cancellations are now a persistent issue and have a serious knock-on effect and point to an extreme lack of capacity in the system.
Each year, HSE hospitals manage over four million planned inpatient and day case procedures.
At the Patient Solutions conference in Dublin, the head of the Department of Health was asked about cancelled procedures.
Robert Watt, Secretary General at the Department of Health, said it depends on what happens when a procedure is deferred, and how quickly the appointment will be rescheduled.
It could be two weeks later, he added.
Mr Watt said that the key issue is for patients to access care as quickly as possible.
He said that for the first nine months of this year, the service has delivered 5.8 million episodes of care, compared to 4.8 million five years ago.
He told the meeting that the number of day cases is up 13-14% and inpatient cases up 8-9% in activity, so the service is delivering more procedures.
Health officials say that there are many reasons for planned operations being cancelled.
These include a lack of bed capacity, a large number of emergency presentations, staff shortages and also patients not being well enough, or available for the procedure.
Sinn Féin's spokesperson on health, David Cullinane, said that the cancellation of nearly one million hospital appointments in less than four years since January 2022 was a "mark of shame" and showed that government failure is causing chaos in the health service.
He added that managers and doctors are left with little choice but to cancel appointments, when they are faced with overcrowding, understaffing and a lack of protected capacity.
