Hurricane Ophelia's tail end is battering Ireland and parts of the United Kingdom with life-threatening winds, heavy rain and deadly storm surges.
Some 120,000 homes and businesses were without power as the storm ploughed through the region with more expected.
The ex-hurricane battered Ireland and the United Kingdom with torrential rain and powerful winds yesterday. With this number of customers out - it is unprecedented it is going to be a number of days, 10 days, before 5% of customers get their power back.
Several rail lines were blocked today by trees that toppled in gusts of 70mph as the storm lashed Scotland and Northern England, and flights and ferries were cancelled in Northern Ireland, where there were 400 incidents of weather-related damage.
Prime Minister Leo Varadkar advised people to stay indoors.
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The storm winds were due to peak between 1600 GMT and 1800 GMT in Dublin and Galway, two of Ireland's most populous cities, and later on Monday in northern areas.
On Storm Ophelia, she expressed her sympathies for the loss of life - and said the UK Government stood ready to provide support. "At the same time, much of northwest Scotland has been removed".
British media are comparing Ophelia to the "Great Storm" of 1987, which subjected parts of the United Kingdom to hurricane strength winds 30 years ago to the day.
Mrs May said she was "absolutely clear" that it was in the interests of everyone in Northern Ireland to see a fully functioning executive up and running so that local decisions could be made by local politicians.