The Republic of Ireland and Northern Ireland are experiencing some of the most extreme weather the island has seen in decades as the aftermath of Hurricane Ophelia strikes.

Schools and hospitals are closed, many public transport and aviation services have ceased operations and the army has been sent to shore-up flood defenses in what has been called an "unprecedented storm" by Ireland's meteorological office.

Two people were killed in separate incidents when trees fell on their cars — a woman in her fifties in the southeast and a man on the east coast. Another man in his thirties died while trying to clear a fallen tree in an incident involving a chainsaw.

Over 360,000 homes and businesses were without electricity with another 100,000 outages expected by nightfall, Ireland's Electricity Supply Board said, describing it as an unprecedented event that would effect every part of the country for days.

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Advising the public to remain indoors as part of its "Status Red warning," it said the worst of the winds would be felt in the west and southeast Ireland Monday morning but would "quickly extend to the rest of the country this (Monday) afternoon."

The extreme weather is the tail-end of Hurricane Ophelia which was downgraded to a tropical storm and on Monday morning was declared a "post-tropical cyclone" by the Florida-based National Hurricane Center. Still, hurricane force winds of as much as 90 miles per hour could occur.

Met Eireann also warned of spells of heavy rain or thunderstorms and storm surges along some coasts that could lead to flooding. All unnecessary travel should be avoided as the storm is passing as the government anticipated hazardous driving conditions and power outages.

Irish Prime Minister Leo Varadkar tweeted on Sunday that defence forces were being deployed to the areas where the worst of the weather was expected.

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He said on Monday that public safety was the main concern

The Irish Republic's Housing Minister Damien English said that the country was preparing for the "the worst weather event for over 50 years," in an interview with the BBC on Monday.

Dublin Airport, in the east of the country, said early morning flights are operating as normal but up to 130 flights to and from Dublin have been cancelled later Monday due to the expected impact of Ophelia.

Aer Lingus, Ryanair, British Airways, Qatar Airways, Air France, CityJet, Emirates and KLM have all cancelled some services. The airlines in question have contacted passengers directly in relation to any services that have been affected.

Meanwhile, the U.K.'s Met Office had an amber warning in place on Monday, warning that parts of Northern Ireland, west Wales, northern England and Scotland could be hit by "a spell of very windy weather associated with ex-Ophelia."

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"Longer journey times and cancellations are likely, as road, rail, air and ferry services may be affected as well as some bridge closures. There is a good chance that power cuts may occur, with the potential to affect other services, such as mobile phone coverage. Flying debris is likely, such as tiles blown from roofs, as well as large waves around coastal districts with beach material being thrown onto coastal roads, sea fronts and properties. This leads to the potential for injuries and danger to life," the U.K. Met Office said on its website.

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- Reuters contributed to this report.