The charity said Government regulations prohibiting the rental of sub-standard bedsits put people on low income at risk.
Director of advocacy Mike Allen said a third of tenants living in a traditional bedsit, often with a shared bathroom and no kitchen facilities, are unemployed.
“Many landlords will not be able to afford to upgrade their properties in order to meet the new standards,” Mr Allen said.
“If they do not upgrade, this means tenants in these properties will have to leave.”
He said while Focus Ireland supported the Government’s aims of improving living standards, there was a concern that a number of vulnerable tenants would be unable to find affordable alternatives.
“We know through our work that people living in bedsits are usually on very low incomes, so we would be very concerned that tenants could be at serious risk of becoming homeless as regulations are implemented unless steps are taken to protect them,” Mr Allen said.
Figures from the 2011 Census show that more than 6,000 people were living in 4,475 privately-rented bedsits – three quarters of which were in Dublin.
Focus Ireland has urged local authorities to support people who might be affected by the new regulations.
It said councils must make a concerted effort to inform tenants of the measures and provide them with support if landlords decide to pull out of the rental market.