More than 48,000 NI homes are without electricity on Sunday evening as Storm Isha causes severe disruption.
NIE said the storm had caused widespread damage to Northern Ireland’s electricity network.
In the Republic more than 170,000 premises are without power.
The extreme weather is putting “significant pressure” on the 999 system, police have said, and a “full multi-agency response” is under way.
Flights and ferry sailings have been cancelled and road travel affected, with Londonderry’s Foyle Bridge closed and fallen trees on the M1.
A ferry due to arrive in Belfast from Birkenhead at 18.30 GMT, remained in the Irish Sea off the County Down coast at 23.30 on Sunday night. Stena Line said its arrival was delayed due to “adverse weather conditions”.
A yellow weather warning for high winds began at noon on Sunday. An amber warning, the second highest level, came into force from 18:00 GMT as the storm increased in severity.
Severe gusts of more than 80mph have been recorded at Magilligan in County Derry on Sunday night.
Winds of 85mph have been recorded on the west coast of Ireland.
The amber alert for Northern Ireland remains in place until 06:00 on Monday.
It means an increased risk of power cuts, damage to buildings, travel disruption, injuries and danger to life because of large coastal waves.
The Met Office has said there is a small chance of peak winds of 90mph (145 km/h) in exposed coastal or hilly areas during this period.
The yellow wind warning will also expire at noon on Monday.
Escalated response
Northern Ireland Electricity (NIE) said it has escalated its response to Storm Isha, with engineers “actively working to restore power after severe weather conditions”.
At present approximately 48,500 customers are without power, but this number could rise as the storm progresses, NIE said.
Due to the extent of the damage and the adverse weather conditions, many customers currently impacted will remain without supply overnight.
NIE has said its engineers are “prioritising hazard and extreme faults” on Sunday night.
“Hundreds of extra staff have been brought in to help reinforce our normal capability. Our Incident Management Centre at Craigavon has been opened, as have Local Incident Centres in locations across Northern Ireland to allow our teams to co-ordinate a localised response,” a spokesperson for NIE Networks said.
“We have mobilised additional call agents to deal with any queries customers may have regarding the repair process in their area.
“We would like to remind customers that if they should experience a power cut they can report the fault online at www.nienetworks.co.uk or call the NIE Networks Customer Helpline on 03457 643 643. Customers can also follow us on social media for regular updates.”
In the Republic, ESB Networks said more than 170,000 homes, farms and businesses were without power, with the expectation of further power outages.
All Aer Lingus flights between Belfast City Airport and Great Britain were cancelled for Sunday evening.
The decision affected 19 flights due to land or take off at the airport.
All British Airways flights from 18:00 GMT were also cancelled, affecting five services between Belfast City and London airports.
As of 21;30, Dublin Airport said 148 flights had been cancelled and 35 diverted to other airports, including four to Belfast International on Sunday afternoon.
The disruption has affected thousands of passengers.