The Simon Community is warning that only 7% of rental properties are within the limits of the rent supplement or housing assistance payments (HAP) 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 measures such as the closing of eviction loopholes and more security of tenure.
According to the study, no properties within rent supplement or HAP limits were found in four out of eleven 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, explained: "Supply across the private rental is a real cause of concern.
"Why we're particularly concerned is for people on low incomes, and people who are in receipt of benefits."
She added: "How they can compete out there in the open market... it's really challenging. What we're finding is sometimes landlords are looking for three months' rent in advance."
The latest figures come in the wake of the latest homelessness figures, which showed that there were 9,104 people in emergency accommodation in January.