The Simon Community is calling on the Minister for Social Protection to increase rent supplement on the back of the latest figures from Daft.ie.
The number of available rental properties has dropped to its lowest point since 2006, according to the latest report by Daft.ie.
It also shows there were a third less properties to rent at the start of 2015 compared to this time last year.
Rents are also up - with the national average rent between January and March this year at just over €960 - an increase of 8.2% on the same period in 2014.
Rent inflation in Dublin has eased, but in commuter counties it is now running at 14%, and economist with Daft.ie Ronan Lyons says the high cost of construction is reducing overall housing supply.
However, the spokesperson for the Simon Community, Niamh Randall, is critical of the Government's action on rent supplements.
She said: "Some of the measures that we ahave seen recently are very worrying, for example, the decision by the department for Social Protection and Tánaiste Joan Burton not to increase rent supplements is really worrying.
"We know that this is causing homelessness, so again we make the urgent call to increase rent supplement limits.
"And this, coupled with measures around rent certainty, would allow more security of tenure for rent in the private rented sector."