10,271 people were in emergency accommodation in January, according to the latest homeless figures from the Department of Housing.
It marks an increase of 540 people from the previous month.
In January, 6,697 adults and 3,574 children were living in emergency accommodation.
Dublin was the region with the highest rate of homelessness, with 4,600 homeless adults.
It comes after the figures dropped to below 10,000 for the first time in nine months in December.
The Simon Communities said the latest figures show a "mammoth amount of work" still has to be done.
Spokesperson Wayne Stanley said: "Over the past 12 months, there has been some slowing of the increase in the number of people in emergency accommodation.
"However, the broader picture shows a 267% increase in homelessness since January 2015, and 3,574 children are in homelessness today. "
