Two new areas - Drogheda and Greystones - have been designated as Rent Pressure Zones (RPZ).
It comes as new figures from the RTB show that average rents across the country increased by 6.6% in the 12 months up until the end of June.
The board's latest report shows that the average national rent in the second quarter of this year was €1,017 - up from €954 in the same period last year.
After a very minor increase in the first quarter of the year, rents increased by 2.9% between April and June.
In Dublin alone, rents were up 3.3% compared to the previous quarter - meaning rents in the capital are 10.8% above their previous peak in late 2007.
The data is based on 19,000 new tenancies registered with the RTB during the second quarter of this year.
Rent Pressure Zones
Launching the report this afternoon, Housing Minister Eoghan Murphy confirmed that Greystones and Drogheda will be designated Rent Pressure Zones from midnight - bringing the total amount around the country to 21.
In designated zones, rents cannot be increased by more than 4% a year.
The Housing Minister suggested there is anecdotal evidence to suggest some landlords are ignoring the rules, as are some tenants desperate for accommodation are willing to pay over the legal limits.
However, he argued that measures such as the Rent Pressure Zones "take time to bed down" - and stressed they will be modified as needed.
Minister Murphy said: "While these figures provide some signal that the RPZs are having an effect in moderating rent increases, it’s not yet clear whether these measures are fully achieving their desired effect.
"Nor can we be complacent, with rents in Dublin now standing almost 11% above the previous peak in 2007. Outside the capital, rents are 7% below their 2007 peak, but this gap is closing."
Deposit Protection Scheme
The minister also confirmed landlords will only be allowed to ask for a maximum of one month's rent as a deposit.
The move comes following reports that one of Ireland’s largest private landlords was forcing some tenants to pay two months deposit – alongside the first month’s rent – in order to secure accommodation.
There will be new regulation on short term lettings like AirBnB, and increased powers for the Residential Tenancies Board (RTB) including the ability to prosecute landlords who implement unfair rent increases.
Minister Murphy said the beefed up RTB will give greater protection to tenants.
“A Deposit Protection Scheme will be established, operated by the Residential Tenancies Board, to handle deposits and to manage disputes efficiently so that decisions are delivered and money is returned quickly,” he said.
“Under this new scheme the RTB will also be able to define a deposit at one month’s rent.
“We won’t be waiting until 2019 for the RTB to take on these enhanced roles- rather additional powers and functions will be rolled out in the intervening period according to priority.”
The deposit protection scheme is already provided for in the Residential Tenancies (Amendment) Act 2015 and a number of charities, including Threshold, have called for it to be commenced as a matter of urgency.
"Ongoing pressure in the rental sector"
Rosalind Carroll, Director of the RTB, explained: “The findings for the second quarter of this year are a further reflection of the ongoing pressure in the rental sector as demand continues to outstrip supply, and with two further areas meeting the RPZ criteria."
Ms Carroll also urged tenants to not accept rent increases greater than those laid down in law.
She observed: "We would encourage any existing, or new tenants, who are faced with increases over and above the 4% cap to refer a dispute to the RTB, and the same advice applies to tenants entering a new tenancy.
"Even if a tenant has agreed to a rent in excess of the limit and signed a tenancy agreement, they are still protected under the law; they cannot contract out their rights."
She added: "If a landlord has been found not to have not adhered to the limits, it can have significant consequences and damages of up to €20,000 can be awarded as well as repayment of the additional rent."