Torrential rain caused severe flooding throughout Northern Ireland and the Irish Republic.
The newly-built Broadway underpass road in Belfast was under 15 feet of water on Saturday as Dublin authorities put a crisis management team on standby.
Numerous major roads across the island were closed to traffic while a train derailed in Co Laois after downpours there sparked a landslide onto the track.
Several sporting fixtures on both sides of the border had to be called off.
The Police Service of Northern Ireland said rivers had burst their banks, two bridges collapsed and 37 major roads were shut down throughout the day.
The main M1 motorway was closed at one stage for Belfast-bound traffic from Sprucefield.
Police urged motorists not to drive anywhere unless the journey was extremely urgent.
"The floods are all over Northern Ireland," said a police spokeswoman. Portglenone and Aghadowey are practically cut off, it seems.
"There have been several landslides, including one about one mile from the Spelga Dam on the Slievemaman Road in Newcastle, Co Down."
Newcastle Town Hall and the town's Presbyterian church were opened as shelters for those affected by the floods, as was Watty Grahams GAA club in Maghera, Co Londonderry.
