The storm also left hundreds of thousands without power across the island. Strong winds ripped the roofs off buildings in Ireland’s largest cities, Dublin and Cork, and pushed seawater over coastal defenses in the western city of Galway.
The national police force, An Garda Siochana, said Monday afternoon that the storm would “bring further violent and destructive winds” and flooding that would endanger life and property throughout the night.
“This is a national red alert,” Prime Minister Leo Varadkar said at a news conference on Monday. “It applies to all cities, all counties and all areas.”
Mr. Varadkar said the last time a storm this powerful had hit Ireland was in 1961 when Hurricane Debbie left 11 people dead.
Winds reached 176 kilometers, or 109 miles, an hour at Fastnet Rock, the country’s most southerly point, the weather service said Monday morning. And the storm’s impact was felt as far away as London, where the sky turned a smoky shade of orange from dust from Sahara sandstorms and wildfires in Portugal and Spain carried north by Ophelia’s powerful winds.
Ophelia, classified as a Category 3 hurricane over the weekend, was downgraded on Monday to a post-tropical cyclone by the National Hurricane Center in the United States.
Nevertheless, Met Eireann said it was the most powerful storm ever recorded this far east in the Atlantic. It was the 10th hurricane of the 2017 Atlantic storm season.
The storm churned north across Ireland on Monday and was expected to move toward Britain late Monday or early Tuesday, according to Britain’s national weather service, the Met Office, which called the storm “ex-Hurricane Ophelia.”
Ophelia’s impact was already being felt in Britain. Schools were closed in Pembrokeshire in southwestern Wales, and flood warnings and alerts were issued for the northwestern and southwestern coasts of England.
Ireland was mostly shut down on Monday as the storm made landfall in the southwestern counties of Cork and Kerry around 7 a.m., lashing coastal towns with heavy rain.
The national electricity provider, the Electricity Supply Board, said an estimated 385,000 customers were without power by Monday afternoon, roughly 17 percent of the population. The utility said it could take several days to restore service.
Schools, universities, courts and hospital outpatient facilities were all closed on Monday. Richard Bruton, Ireland’s education minister, said schools would remain closed on Tuesday “in the interests of child safety.”
Public transportation, ferries and flights were canceled, and people were advised to stay indoors.
Shane Ross, Ireland’s transportation minister, said at a news conference on Monday that even after the storm passed, “The roads will not be safe.”
“There will be flooding, and it will be very dangerous,” he said. “People should be very careful this evening if they are driving.”
The national police force tweeted photographs of trees blocking roads and urged motorists to travel only if necessary.
It also tried to inject some humor into the situation, sharing a cartoon that depicted the island blowing off the map. “Gusty winds can move us all in mysterious ways,” it said.
Although Monday was nominally a working day, the country’s main business association, the Irish Business and Employers Confederation, or Ibec, encouraged its members to allow employees to work from home if possible.
“Given the current red status weather warning and widespread issuing of public safety alerts, Ibec encourages all businesses to minimize the movement of employees tomorrow,” the group said in a statement on Sunday. “Safety should be of the utmost priority.”
Mr. Varadkar told reporters he was concerned that “people may believe that the storm isn’t going to be as bad as predicted.”
“There is a possibility that we are going to be here tomorrow relieved that the damage was less than we thought, but we can’t operate on that basis,” he said. “So I don’t want anyone to think that this is anything other than a national emergency and a red alert in all counties, all cities, all areas.”