The tough economic climate has created a "homelessness timebomb", a leading campaigner has warned.
In addition, freezing temperatures that hit a November record of minus 10C have sparked concerns for the homeless. But charities battling the issue in Ireland north and south have signalled that the developing economic crisis is making their job harder.
In Northern Ireland a campaigner warned that rising unemployment and cuts in welfare benefits were a "timebomb", while in the Republic a key charity reported a drop in funding.
There are 5,000 people in the Republic in temporary accommodation, with even more on social housing waiting lists and at least 60 people sleeping rough in Dublin alone.
In Northern Ireland, where charities are currently marking Homeless Awareness Week, experts said 10,000 were legally homeless and in temporary accommodation, but welfare cuts could force more on to housing waiting lists or even the streets.
The news has coincided with the arrival of a dangerous decline in weather conditions. In the Republic, Met Eireann said night-time temperatures fell to minus 10C or even lower. North of the border, the lowest reading was at Lough Fea in Co Tyrone, which recorded minus 9.5C.
Ricky Rowledge, of the Northern Ireland Council for the Homeless, said nearly 19,000 have presented themselves as homeless in the region, with half of those legally recognised as such, and it was getting harder to secure temporary accommodation.
"People having to sleep on the street in Northern Ireland is not a major problem in terms of numbers, but those that are, are in dire circumstances," she said.
Pointing to cuts in housing benefit and a drop in employment opportunities, she added: "This is potentially a timebomb in homelessness. A poster campaign once said people were only three pay packets away from homelessness, but it could get worse."
She said rises in private rents and a fall in welfare support could make it easier to become homeless. "We do need to protect those who are on the streets in this cold weather," she said. "But everybody needs to be aware that it could be them."