The Simon Communities are warning 93% of rental properties are beyond the reach of people availing of the HAP or rent supplement schemes.
The charity is publishing a study this morning showing the effects of Ireland's current rental market on vulnerable people.
They are concerned about the lack of supply and want to see things like the closing of eviction loopholes and more security of tenure.
According to the study, no properties with rent supplement or HAP limits were found in four out of 11 study areas, with just one property available in two other areas.
The overall percentage of properties within the limits is the lowest level recorded since the increase in limits in July 2016.
Niamh Randall, national Spokesperson for the Simon Communities, said: "Supply across the private rental sector is a real cause for concern, and why we are particularly concerned for people on low incomes and people who are in receipt of housing benefits like rent supplement and the Housing Assistance Payment.
"How they can compete out there in the open market, it's really challenging.
"What we are finding sometimes is that landlords are looking for three months rent in advance, we're hearing calls for rent money."