Some 2,400 children and 1,200 families are without a roof over their head, Focus Ireland said as it made an urgent plea for help.
Sister Stan Kennedy, who launched the charity's Christmas appeal at Government Buildings, said: "It breaks my heart to think that up to 2,500 children will be homeless on Christmas Day this year."
It features TV, radio and outdoor advertisements and features a mother and her child alongside the stark line: "Homelessness. We can't live like this any more."
Sr Stan (pictured) added: "I know from meeting families who are homeless that we support, it's the children who feel it the worst.
"Many times, a family who are homeless are often squeezed into one hotel room - three or four people in one room, nowhere to cook or for children to play.
"There are also many single people and couples homeless and they all need a place to call home."
Focus Ireland's family team services have supported 230 families and just under 450 children in Dublin to secure a home and escape from the trauma of homelessness in the first 10 months of this year.
The charity also supports hundreds of families who are still homeless and living in hotels or bed and breakfast accommodation.
One mother, Gillian, who was homeless but has now secured a place to live, said: "When you have a home you don't realise how lucky you are.... my daughter was only two weeks old when we became homeless.
"I don't think I'd even be here now if it wasn't for Focus Ireland. I wake up every day now and think 'I have a home."
Members of the public are being asked to donate what they can afford via www.focusireland.ie or by telephone on 1850 204 205.