It's claimed there could be around 100 thousand rental properties in Ireland that are not registered with the RTB.
It comes amid reports of increasing numbers of landlords leaving the market.
All private residential landlords are obliged to register their tenancies with the Residential Tenancies Board.
Chairperson of the Magazine Road Residents Association in Cork. Catherine Clancy, says many are choosing not to register:
"It is our lived experience that not alone are a third of the properties in our area unregistered on an ongoing basis with the RTB, but we believe that these figures are reflected nationally and you're talking about 100,000 properties nationally not registered with the RTB."
Meanwhile, Estate Agent Ben Thompson says landlords can rent out properties easily with out needing to register:
"We're very much aware of a black market in rentals where they're not going through agents. "
"It's so easy, there's so much power with anyone who is a landlord. They can just rent it overnight by putting a Facebook post up. "
"They don't need to go through agents and therefore and I think that's where they're avoiding using the RTB."