Large parts of Newry in Co Down have been swamped by floodwater as the city’s canal burst its banks amid heavy rainfall.
Dozens of businesses were engulfed in the floods, with widespread damage caused to buildings, furnishings and stock.
Police have warned people to stay out of the city centre as streets have turned into rivers.
Thousands of sandbags have been stacked along the canal to try to stem to flow amid fear of further breaches.
Parts of the canal walls have already collapsed into the water and the authorities are concerned more sections could give way.
Other areas in the north east of the island of Ireland were also badly hit by the overnight rain, with homes flooded in Camlough in Co Armagh and Newcastle in Co Down.
Just across the Irish border in Co Louth, a bridge close to Riverstown partially collapsed amid rising water on the Cooley Peninsula. The town of Carlingford also experienced some flooding.
The main motorway and train line linking Belfast and Dublin were both affected by flooding on Tuesday, causing major disruption for cross-border travellers.
Further south, flooding has also damaged homes in parts of Co Wexford this week, including the town of Rosslare.
In Newry, Sugar Island, Kildare Street, Canal Quay and part of Bridge Street were among the areas worst hit.
Paul McCartan, who co-owns McCartan Bros, a clothing shop on Sugar Island, arrived at the business late on Monday to find it under three feet of water.
Stock was floating around in the shop on Tuesday.
“It’s an absolute mess,” he said.
“I came up this morning hoping the waters would be lower, and if anything they were far higher. I needed to get in, I have stock in there, it’s my livelihood and unfortunately the inside of the shop is an absolute mess.”
Newry resident Desy Heatley described the scenes as a “repeat of what happened here around 35 years ago”.
“We were up and down here in boats,” he said.
“It’s pretty bad, I’m sure all the shopkeepers are raging, we’d no warning or nothing.”
Police Service of Northern Ireland District Commander Superintendent Norman Haslett said on Tuesday: “We are continuing to warn the public to avoid Newry city centre and the surrounding area because of excessive flooding due to the canal bursting its banks overnight and heavy rainfall.
“I would also urge people to keep away from the canal and the river at this time due to the hazardous conditions caused by high water levels and the fast flowing current.”
Counties Down, Antrim and Armagh were covered by an amber rain warning over Monday night.
The deluge happened ahead of the arrival of Storm Ciaran, which is expected to hit the UK and Ireland on Wednesday.
A yellow warning for the whole of Northern Ireland has been issued from 9pm on Tuesday to 9am on Wednesday.
A further yellow warning for rain will come into effect at 6am on Thursday for counties Down, Armagh and Antrim. It will lift at midnight that night.
A 24-hour yellow rain warning from Met Eireann for Co Kerry in the south west of the island came into effect at 12pm on Tuesday.
Met Eireann has also issued a yellow rain warning for counties Carlow, Kilkenny, Wexford, Wicklow, Cork and Waterford from 7pm on Wednesday to 7am on Thursday.
A yellow wind warning for Clare, Kerry and Galway will be in place from 5am to 11pm on Wednesday.
Irish premier Leo Varadkar said flooding is “becoming more frequent and becoming more severe” in Ireland.
Speaking to reporters in Co Kildare on Tuesday, the Taoiseach said: “There isn’t really anything new about floods, floods have been happening for millennia, but it is evidence that flooding is becoming more frequent and becoming more severe.
“And we saw a real, very obvious example of that in Cork in the last couple of weeks and now Louth and Wexford and also in Newry today.”
Across the UK the Environment Agency has issued more than 25 flood warnings ahead of the arrival of Storm Ciaran on Wednesday, while the Met Office has issued yellow weather warnings for rain that lasts until Thursday.
Gusts of 80mph are possible along the south coast of England, with 20mm to 25mm of rain expected across southern and western areas, but potentially up to 40mm to 60mm over higher ground, the Met Office said.
The forecaster advises the latest rain could lead to disruption on roads and to public transport while areas already affected by flooding after Storm Babet could get worse.
