Housing charity Threshold has urged the government to clarify the long-term status of the rent pressure zone legislation.
The rules, which cap rent increases at 4% per annum, are currently due to expire before the end of the year.
A spokesperson for Threshold said it is essential that clarity is given to renters in RPZs.
"As it currently stands, the RPZ status for Dublin and Cork expires on December 24," it said.
"Tenants renting in both of these cities need to know now what rental controls will be afforded to them once the designated status expires."
On December 13, 2016, then-Housing Minister Simon Coveney approved the creation of RPZs in the four Dublin councils and in Cork city.
Speaking at the time, he said, "These are areas where annual rent increases have been at 7% or more in four of the last six quarters and where the rent levels are already above the national average. Rent pressure zones will be designated for a maximum 3 years, by which time new supply will have come on stream and pressures will have eased somewhat in these areas.
With the allocation of RPZ status to Navan and Limerick City East this week, there are now five local authority areas and 16 local electoral areas subject to the restrictions.
However, critics say that the extension of the scheme is too little too late for some areas and others have indicated that the rules have not had the required impact.
Members of Fianna Fáil, Sinn Féin and the Green Party have called for further interventions in the market, while charities like Threshold and the Simon Communities say that more needs to be done.
Even some of those areas that are currently subject to RPZ legislation have continued to experience significant rent increases in recent years.
Since Quarter 4, 2016, when the legislation was introduced, rent in the south-east of Cork city has increased by 13%, according to the RTB's Rent Index. Renters are now paying €1,323.66 and the average rent paid has increased by more than 7% in each of the last six quarters.
In the Ballincollig-Carrigaline municipal district, the average rent has increased by 14.6% since quarter 4, 2016 to quarter 4, 2018. It now stands at €1305.21 per month.
Limerick city east, which was designated a rent pressure zone this week, has seen its average rents increase by 30.68% in that time while the national average has grown by 14.98% from €986.50 per month to €1,134.29 per month.
A spokesperson for the Department of Housing said that the Housing Agency 'continues to monitor the rental market and may recommend further areas for designation'.
However, they were unable to say whether designation would be extended.
"Future policy decisions in relation to the rental sector will be informed by ongoing careful analysis of the rental market. Decisions flowing from this process, including in relation to the future RPZ designations, will be outlined in due course."