Three people have been killed as hurricane-force winds from Storm Ophelia continue to batter Ireland and head towards the UK.
A woman in her 50s died after a tree fell on her car in West Waterford, while her passenger - a woman in her 70s - was injured.
A man in his 30s died in a chainsaw accident in Cahir, Co Tipperary, after trying to remove a tree downed by the storm and the male occupant of a car died when a tree fell on the vehicle in Dundalk, police said.
Gusts of up to 92mph have already hit the south west coast and forecasters warned that worse is yet to come. Ophelia has put Ireland on lockdown and left more than 120,000 homes without power, as parts of the UK brace for hurricane-force winds and the Met Office warns of a potential "danger to life".
Remnants of the hurricane - that was downgraded after making its way across the Atlantic - battered Britain's west coast on Monday afternoon, with gusts of up to 80mph, exactly 30 years after the Great Storm of 1987 killed 18 people.
Fears about the impact of strong winds have seen troops placed on standby and many public services in Ireland and Northern Ireland closed with schools, government buildings and courts among the services affected.
Planes were grounded at Manchester Airport, with 20 flights cancelled and passengers warned to check ahead. Ireland experienced the worst of the weather, with schools closed and around 130 flights cancelled at Dublin airport.
Schools and colleges were closed in Northern Ireland, which is covered with an amber weather warning - meaning there is a "potential risk to life and property", issued when forecasters believe people need to be prepared to change their plans and protect themselves from the impacts of severe weather.
The storm began to move across to Wales, northern England and Scotland throughout Monday.
High winds are expected across the entire region, while a yellow warning is in place for much of Wales, Scotland, north east England, north west England, south west England and the West Midlands. Parts of Scotland and Wales have also been upgraded to amber.
Around 200 properties in Wales suffered power cuts, a number of schools closed early and the Cleddau Bridge was shut to high-sided vehicles, Pembrokeshire County Council said.
Flood warnings were also in place along the Pembrokeshire coast, parts of west Scotland, north-west England and Cornwall.
Forecasters warned of flying debris, such as tiles blown from roofs, as well as large waves around coastal districts with beach material being thrown on to coastal roads, sea fronts and properties.
"The centre of Ophelia will be to the south-west of Ireland on Monday morning," Met weatherman Grahame Madge said.
"It's due to come over the west coast of Ireland around midday and as it goes through the day it will be centre over to the north of Northern Ireland.
"It will be gradually easing up into Scotland overnight and into Tuesday morning - it's weakening as it goes.
"Parts of England, areas like the North West, are covered by a warning. The impacts will be felt in northern England into Tuesday."
In Ireland, Met Eireann has issued a "status red" weather alert for Galway, Mayo, Clare, Cork and Kerry, warning of severe winds and stormy conditions.
More than 1,000 troops are on standby to deal with any destruction wrought by the storm, which is now a post-tropical cyclone but is still forecast to bring hurricane-force winds to Ireland and the United Kingdom.
More than 120,000 homes and businesses were left without power as Ophelia hit Ireland.
Taoiseach Leo Varadkar has urged the public to stay safe, saying: "The advice is: stay indoors until the storm passes.
"Whether that is at work, in their home or some other home, stay indoors. Check on neighbours and relatives.
"Bear in mind it is coming your way and it is a national red alert. It is a very dangerous storm. The last time there was a storm this severe 11 lives were lost."
All schools and colleges have been closed, ferries cancelled, court sittings postponed and the Defence Forces put on standby ahead of the severe storm.
Weather forecasters have warned of a potential threat to life and advised the public to stay off the roads and away from the coast during the height of the storm if possible.
"Defence forces being deployed in Red weather alert areas and on standby for further action (on Monday)," Taoiseach Leo Varadkar tweeted. "Please check in with older neighbours and those who need medical care."
Met Eireann described the storm as the most powerful hurricane to have ever been this far east in the Atlantic on record.
It is also predicted to be the most severe weather to hit the country since 1961, when Hurricane Debbie made landfall.
Timmy Flaherty, 72, who caused an outcry when he was filmed taking a dip in the Atlantic as the eye of the storm approached Galway, said he expected the backlash.
"I'd go along with them alright, but it's up to each individual. Whatever turns you on," he said.
"You're safe enough in the water - if you don't panic. The secret of the whole thing is not to panic."
Mr Flaherty, from Galway, who never misses a day in the sea, caused local police officers to sound sirens and urge people off the promenade as baffled onlookers took photographs.
The US National Hurricane Centre said the storm could bring 50 to 75 millimetres of rain in western Ireland and Scotland, with coastal flooding and "large and destructive waves" where it makes landfall.
Met Office forecaster Luke Miall said that while storms with these wind speeds tend to happen at this time of year, the one on its way is "quite a substantial system", adding that he would describe it as "pretty exceptional".
Mr Miall said that while Ophelia would no longer be a hurricane, it would still bring "hurricane-force" winds.
Meanwhile, airports are advising passengers in Ireland to check the latest information, with a number of Aer Lingus flights cancelled due to severe weather and the prospect of further cancellations with other carriers.
Dublin Airport and Shannon Airport was advising passengers to check the latest flight information before travelling to the airport, while Cork Airport said cancellations were likely.
A Ryanair spokesman said: "We will inform customers in the event of any changes to our flight schedule and the latest flight information will be posted on the Ryanair.com website."