The number of people awaiting outpatient appointments in public hospitals rose in December to 516,162, erasing a slight decrease recorded the previous month.
In November, the figure had fallen to 515,360 from 516,363 the previous month.
The number of people waiting for more than nine months to be seen has also risen slightly - from 196,297 in November to 196,472.
The number on the inpatient, day case waiting list dropped from almost 71,000 in November to some 70,200.
Fianna Fáil said the latest numbers were unacceptable and required an urgent remedy.
Updated monthly data on the public health system are released by the National Treatment Purchase Fund (NTPF).
Minister for Health Simon Harris, who routinely responds to the NTPF publication, welcomed the drop in inpatient numbers - which have fallen from a peak of 86,100 in July 2017. However, he acknowledged that "significant work" was required to address waiting lists.
‘Priority’
“Improving access for patients is a key priority and today’s publication highlights the continued progress that is being made by the HSE and the NTPF in delivering on this priority,” he said.
He noted that long-term waiting times had dropped in certain specific treatment areas. The number waiting more than nine months for a cataract procedure has fallen by 87 per cent since July 2017, while the number awaiting tonsillectomies is down 84 per cent.
The Department of Health has acknowledged the outpatient waiting list remains a "significant challenge" and said a key priority for this year is to improve access.
Fianna Fail's health spokesman Stephen Donnelly noted that 152,940 of those waiting for outpatient appointments had been doing so for more than a year.
"Such long waiting times are simply unacceptable and urgently need to be addressed. The fact is that the number of people waiting over a year is greater than the entire population of Wicklow, " he said.
Mr Donnelly also accused Mr Harris of issuing a “complacent response” to the latest numbers and said outpatient waiting times had increased by a quarter during his time at the helm of the department.
