The Health Service Executive's chief clinical officer Dr Colm Henry said they are working closely with hospitals and communities to ensure patients with the greatest needs are prioritised.
"We’ve also been working closely with the INMO in seeking exceptions, again to make sure those cases with the greatest need are exempted from industrial action.
Dr Henry paid tribute to staff "who are working in extremely difficult circumstances."
Over 37,000 nurses belonging to the Irish Nurses and Midwives Organisation resumed their strike action this morning in a dispute over pay and staff shortages.
It is the third day on the picket line with no sign of a breakthrough in the escalating dispute.
Once again, all outpatient, inpatient and day surgery appointments are cancelled, as are routine community nursing services and health centre nurse clinics.
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The number of hospital and community health service appointments cancelled as a result of the nurses' strike so far has reached nearly 80,000 according to new figures from the HSE.
The Health Service Executive said that today, more than 27,000 appointments have been cancelled.
The figure involves 2,000 planned procedures, 13,000 outpatient appointments and about 14,000 community appointments.
Three further consecutive strike days are scheduled for next week, which Dr Henry said: "is much more of a headache.
"With one day of strike action, we are able to absorb all the extra cancellations into further normal working days, and give people assurances that we will reschedule them in a number of weeks.
"As the strike days accumulate, we have serious concern looking forward into next week, about the impact this will have on services and on patient safety."
Dr Henry said as the strike days progress, the accumulated number of patients cancelled grows greater and greater "and our ability to reschedule those patients in an acceptable time interval is severely compromised."
As his speciality is geriatrics and the care of older people, Dr Henry said his concern is "access to services they need. Not just in hospitals but in the community, where older people rely on public health nurses."
He said he is "concerned" for older people who rely on community services as well as people living with a disability.
The HSE is giving priority to those whose appointments and surgeries have been cancelled, according to Dr Henry.
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