Thousands of Co Dublin council homes with four and five bedrooms have just one occupant, figures show.
More than 12,700 bedrooms in houses and apartments owned by the four Dublin local authorities are empty, while almost 10,000 people are living in emergency homeless accommodation.
Some 195 four and five-bedroom homes in the city are occupied by just one person, the figures show.
At the same time, larger households are forced to live in overcrowded conditions, with 1,914 households with six or more occupants living in a one, two or three-bedroom home.
The information comes as the latest homeless figures show there are 5,869 adults and 3,829 children classed as homeless, up by 171 last month compared with August.
The number of families presenting as homeless is down, but information provided to Karl Deeter, of mortgagebrokers.ie, and Brendan Burgess, of askaboutmoney.com, highlights how many homes are not being fully utilised.
Just over 21pc of public housing stock owned by councils and rented to tenants is under-occupied.
The highest rates are in Dun Laoghaire Rathdown and South Dublin at 26pc.
In Dun Laoghaire, 1,187 of its total stock of 4,553 units is not fully utilised. In South Dublin, 2,409 of the 9,274 units have more bedrooms than occupants, with 625 homes with three or more bedrooms being occupied by one person.
The ratio drops in Fingal to just over 21pc, or 1,290 units out of 6,031. There are 355 three and four-bed homes occupied by just one person in one of the fastest-growing local authority areas in the State.
The lowest rate is in the Dublin City Council area at 19pc. The country's largest local authority owns 25,979 units, with 5,002 under-occupied.
Mr Deeter said that families should be forced to downsize when the property was too big for their needs.
"Why should we spend billions of euro building homes when we could resolve almost all of the homelessness problem through the cheaper, faster and fairer reallocation of existing stock," he said.
"What's wrong with ending the suffering of the homeless and those in overcrowded accommodation by using homes we already have?"
Councils cannot compel people to downsize.
In addition, adult children have succession rights allowing them to take over the property if the main tenant dies.
"Effectively a house is for life," one council source said.
"There's very little incentive for people to move. We cannot force people out. Other European countries wouldn't be as soft as us and [low] rents are an encouragement for people to stay."
The Department of Housing said it was examining how housing supports were assessed and units allocated, and that changes might be made after "policy research".
The Government plans to spend €5.85bn creating almost 50,000 new social homes by 2021, of which 33,500 will be new-build.