Focus Ireland has said that, given the current rate at which the State is housing homeless people under a fast-track scheme, it would take over two years to house the existing numbers who have lost their dwellings.
The calculation assumes that no families become homeless in the meantime.
The prediction was made by the housing charity's head of advocacy, Mike Allen, at an international conference on the issue in Dublin.
He also told delegates that one of the most significant moves by the Government to tackle the current homelessness crisis was Minister for the Environment Alan Kelly's decision to ensure that half of all available social housing was allocated to homeless households and other vulnerable groups.
When the new policy came into force at the end of last January, all 232 homeless families in the four Dublin authority areas became eligible to be fast-tracked into social housing.
After five months, he said, 134 of these families remained in emergency accommodation while 97 families had been successfully housed.
Mr Allen said the outcome "made an enormous difference" to the 97 families and will save the Dublin Regional Homeless Executive around €6m that would otherwise have to be spent on renting hotel rooms as emergency accommodation.
However given that 78 new families became homeless in August alone, the steps were inadequate, he said.
He calculated that "even if we could completely stop the flow of new families" into homelessness today "it would still take a staggering two-and-a-half years until the last family was housed - given the current rate at which we are able to provide homes for people".
The State has made Focus Ireland's family team responsible for assisting all homeless families in the capital's four local authority areas.
Mr Allen was addressing the conference organised by FEANTSA, the European network of homeless organisations of which he is president.