The north of the country is experiencing some of the worst October weather for years, with landslides causing havoc on the province's roads, and home-owners erecting flood defences.
The Meteorological Office and the Police Service of Northern Ireland issued warnings to motorists after a second night of heavy rain.
The Roads Service was also supplying sandbags to houses at risk from flooding.
Some of the worst flooding was in Co Down, where water flowing off the Mourne Mountains and a high tide along the coast was causing considerable flooding. Sections of the road between Annalong and Newcastle were affected by mudslides, and the Omeath road in Newry, is closed to traffic.
In the port of Larne, north of Belfast, the picturesque Co Antrim coast road was also affected by a landslide at the Black Arch between Branch Road and Newington Avenue.
Flooding closed Troopers Lane in Carrickfergus, and also hit the Hightown Road and Corrs Corner areas of Glengormley.
Roads Service spokesman John White said staff were working throughout the night to deal with the flooding. "In some cases they've had to close roads and, indeed, have been sandbagging properties to avoid flooding," he said.
"It's been particularly bad in the Newcastle area, where the main emphasis has been on trying to keep the roads open."
Firefighters were also working throughout the night. A Fire Service spokesman said the flooding was the worst he had ever seen. "Driving along the road there were manhole covers popping out off the road," he said.
The rain is expected to give way to gale-force winds later today.