Patients in emergency departments will face longer queues from next week in several hospitals as 150 junior doctor posts remain unfilled.
Hospital emergency departments in small to medium hospitals will be worst hit with a shortage of registrars who are experienced doctors.
It will mean many patients will face delays before getting access to a senior decision maker as most urgent cases are dealt with.
Several hospitals are transferring junior doctors from other sections of the hospital and this will also lead to pressure as they are unfamiliar with emergency medicine.
The Health Service Executive (HSE) said all hospitals had contingency measures in place and the aim was to minimise the impact on patients as much as possible.
"The number of vacancies will continue to reduce over the coming days and weeks as doctors from India and Pakistan, following a major international recruitment drive, arrive in Ireland," a spokesman said.
"Over 200 of these doctors are currently going through visa and registration with the Medical Council and will take up posts once these arrangements are in place."
The HSE is also to hire locum doctors.
A breakdown of where the vacancies are shows emergency medicine and general surgery had the highest, followed by paediatrics and anaesthesia.
Paediatric services in the north east will have to be curtailed, leaving children waiting longer for outpatient appointments.
It will mean a reduction in the number of outpatient clinics, which will push the waiting time for routine appointments from 16-32 weeks.
Junior doctors across the country will begin their new six-month stints from next Monday but several hospitals have failed to recruit enough.
The HSE said it costs €960,000 to train a junior doctor from internship to appointment as a consultant.