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.
More than 360,000 homes and businesses were without electricity on Monday with another 100,000 outages expected by nightfall, Ireland's Electricity Supply Board said, describing it as an unprecedented event that would affect every part of the country for days.
Around 170 flights from Ireland's two main airports at Dublin and Shannon were cancelled.
Meanwhile in London, skies were turned yellow and the sun red as dust from the Sahara got caught up in the storm and washed over southern parts of the UK.
Two people were killed in separate incidents in Ireland when trees fell on their cars - a woman in her 50s in the southeast 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 after 10:40am (local time), the Irish National Meteorological Service said, with winds as strong as 190km/h hitting the most southerly tip of the country. Coastal flooding was likely.
"This storm is still very active and there are still very dangerous conditions in parts of the country. Do not be lulled into thinking this has passed," the chairman of Ireland's National Emergency Co-ordination Group, Sean Hogan, told national broadcaster RTE.
The armed forces were sent to bolster flood defences, public transport services and hospitals were closed and schools across Ireland and Northern Ireland will remain shut for a second day on Tuesday.
Hundreds of roads were blocked by fallen trees, Hogan said. Photos on social media showed roofs flying off buildings, including at Cork City soccer club's Turner's Cross stadium where the roof of one stand had collapsed.
Prime Minister Leo Varadkar advised people to stay indoors. The transport minister said it was not safe to drive.
The storm winds were due to peak between 5pm and 7pm local time in Dublin and Galway, two of Ireland's most populous cities, and later on Monday in northern areas.
Britain's meteorological service put an Amber Weather Warning into effect for Northern Ireland from 3pm to 10pm local time, saying the storm posed a danger to life and was likely to cause transport cancellations, power cuts and flying debris.
It is expected to move towards western Scotland overnight and "impactful weather" is expected in other western and northern parts of the UK it said.