Dublin City Council paid private landlords more than €106m last year.
This was an increase of 77% on 2017.
There are two housing-support schemes for social tenants - the Housing Assistance Payment and the Rental Accommodation Scheme - for which the council pays the landlord, while tenants also pay a percentage of the rent.
Details released under the Freedom of Information Act show Dublin City Council paid landlords €18.4m for 1,128 tenancies under RAS last year, with the average payment being nearly €1,400 a month.
The council also paid landlords €88m in 2018 for tenancies under the HAP scheme - up from €40m the year before.
"What's particularly staggering is that subsidising landlords has increased from half a million in 2015 to over €100m in 2018," said Eoin Ó Brion, Sinn Féin spokesperson on housing.
"And a very large portion of that is homeless HAP. That's just the wrong way for money to be spent.
"[It] shows one: the depth of the housing crisis, but two: that the Government hasn't got it's priorities right.
"It should be building more houses and reducing expenditure on subsidised private rental."
The local authority says 2,365 homeless HAP tenancies were set up between 2017 and 2018.