The amount of money paid by the Government to private landlords has spiralled by more than €200m in four years.
Housing Assistance Payments (HAP) are also set to rise substantially in the coming months as the Government looks to the rental sector to ease the housing crisis. HAP is paid to private landlords on behalf of those entitled to housing support.
Figures released to Fianna Fáil's Darragh O'Brien show that in 2014, just €390,000 was spent on HAP payments, with fewer than 500 households on HAP. Last year, €276.6m was paid, with more than 40,000 households being accommodated via HAP.
There were 25,500 separate landlords and agents in receipt of monthly HAP payments, according to the department's most up-to-date data which does not account for the final three months of 2018.
The Government plans to ramp up spending on HAP to €422m this year, with plans for almost 17,000 more households to be added to the scheme.
Mr O'Brien said Fine Gael was "pouring taxpayers' money into the private sector".
"Under Fine Gael, we are seeing a massive rise in HAP spending, rather than investment in direct build. This is a short-term approach that will ultimately cost the State billions of euro," he said.
However, Housing Minister Eoghan Murphy defended the spend and said "HAP is necessary now, but we are coming to a point where we are reducing our dependence on HAP".
He said if Fianna Fáil was "paying attention", it would understand that spending had increased on social housing beyond initial Rebuilding Ireland targets.
"New HAP leases created this year will be down on last year because the stock of social housing is increasing," he said. "In 2021, more people will be accommodated in social housing homes than in new HAP tenancies."