Three people have died as Tropical Storm Ophelia battered Ireland's southern coast, knocking down trees and power lines and whipping up 10-metre waves.
Over 360,000 homes and businesses were without electricity with another 100,000 outages expected by nightfall local time, Ireland's Electricity Supply Board said, describing it as an unprecedented event that would effect every part of the country for days.
Around 170 flights from Ireland's two main airports at Dublin and Shannon were cancelled.
Two people were killed in separate incidents when trees fell on their cars — a woman in her 50s in the south east and a man on the east coast.
Another man in his 30s died while trying to clear a fallen tree in an incident involving a chainsaw.
The storm, downgraded from a hurricane overnight, was the worst to hit Ireland in half a century.
It made landfall with winds as strong as 176 kilometres per hour hitting the most southerly tip of the country and flooding likely.
"These gusts are life threatening. Do not be out there," the chairman of Ireland's National Emergency Coordination Group Sean Hogan said on national broadcaster RTE.
Schools, hospitals and public transport services were closed and the armed forces were sent to bolster flood defences.
Photos on social media showed the roof of a stand at Cork City soccer club's Turner's Cross stadium had collapsed.
Hurricane-force winds are expected in every part of the country, Prime Minister Leo Varadkar said, advising people to stay indoors.
"While the storm in some parts of the country is not yet that bad, it is coming your way," he said.
Ireland's transport minister said it was not safe to drive.
Britain's meteorological service put an Amber Weather Warning into effect for Northern Ireland, saying the storm posed a danger to life and was likely to cause transport cancellations, power cuts and flying debris.
"Impactful weather" is expected in other western and northern parts of the United Kingdom, it said.
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.
The storm is expected to move towards western Scotland overnight.
The Irish Government said the storm is likely to be the worst since Hurricane Debbie, which killed 11 in Ireland in 1961.
It is likely to pass close to an Ireland golf course owned by US President Donald Trump, who has been planning a wall to protect its greens from coastal erosion.
Similar sized storms in the past have changed the shape of stretches of the Irish coastline, climatologists said.