Nearly 620,000 people were waiting for an inpatient or outpatient appointment last month.
The National Treatment Purchase Fund (NTPF) says there were 553,400 patients waiting for an inpatient or day-case treatment.
A further 66,600 people were waiting for an outpatient consultation.
However, the combined figures are down more than 10,000 from November's waiting lists.
In a statement, the NTPF said it received an increase of €25 million in funding in Budget 2020 - bringing the total to €100 million.
The group said: "The additional funding will be used to benefit patients throughout the country, as the NTPF maintains its strong focus on improving waiting times for inpatient and daycase treatment, while also significantly increasing its focus on outpatients.
"The NTPF will also be in a position to support public hospital emergency departments, including by funding diagnostic scans."
Stephen McMahon from the Irish Patients Association said the figures remain far too high.
He said: "It's good to see some movement in a downward direction - but we've got to realise over €75 million has been pumped into the NTPF to get better timely access for public patients.
"Year-on-year - looking at December last year versus today - there are still slight increases, and we have to see that this reduction continues on now."
Mr McMahon added: "Clearly the build-up for some of these patients who've been waiting for years to get treatment is not being helped by the huge overcrowding in our emergency departments."