Hurricane Ophelia has killed three people in Ireland as it pounds the country with some of the most extreme weather it has experienced in decades.

The storm — now officially a post-tropical cyclone — has knocked two trees onto drivers, killing them inside their cars.

A third man, who was trying to clear a fallen tree from a road, inadvertently killed himself with his chainsaw. Police confirmed all three deaths.

Sea defenses have been breached, causing flooding, while powerful winds have ripped the roofs from buildings and knocked down countless trees. Roughly 360,000 people are without power.

The first victim was a woman in her 50s who was killed by a falling tree in County Waterford. The second was the man with a chainsaw, who died in County Tipperary. The third was a male driver killed in Ranvensdale, County Louth.

A toppled tree in Dublin. Twitter/corkcityrecreation‏

Several fallen trees in Cork. Twitter/Dublin Fire Brigade

Powerful winds have been widely recorded, with gusts measured at almost 120mph.

Coastal defences in Galway, on the west coast, were breached and sea water streamed through the streets:

Coastal defences completely breached at Salthill, Galway. The Atlantic now extends to the promenade. #Ophelia pic.twitter.com/qF9B47Ty3i — David Blevins (@skydavidblevins) October 16, 2017

Video on social media also showed roofs being ripped off buildings.

This one shows the nearby town of Passage West:

Roof gone from stand in passage west gaa club. #ophelia pic.twitter.com/mZm4uQ8rAo — Billy OK (@bingobars) October 16, 2017

While this shows a school in the city of Cork:

Cork City's home football ground, Turners Cross, was also seriously damaged:

Here's where Ophelia is expected to travel over the course of the day:

Irish forecasters have warned people in at-risk areas to stay inside their homes to avoid the effects of the storm. Police and the coast guard have made repeated pleas for people to stay away from the coast because of rough seas.

Hospitals across the country cancelled thousands of non-essential procedures because of the bad weather. All schools in Ireland were closed on Monday, and will remain so on Tuesday.

Kerry Airport, which is close to the path of the storm, cancelled all flights on Monday and closed completely. Cork Airport, which is slightly further from Ophelia, remained open but still cancelled dozens of flights.

Wind speed measurements showed extreme highs of 118 mph at Fastnet Rock, a small island with a lighthouse four miles from the Irish mainland. On the mainland at Roches Point, County Cork, the wind speed was measured at 97 mph (156 kmph).

The storm is beginning also to be felt in the British territory of Northern Ireland. The symbolic Peace Bridge near the border between Ireland and the UK was closed due to the bad weather.

This graphic from the US National Hurricane Center shows the likely track of the storm, which will take it across the sea to parts of Scotland:

A map, generated early Monday morning European time, shows the expected track of Ophelia over Ireland and the UK. US National Hurricane Center

In most of England the weather was calm, but a secondary effect of the storm caused the upper atmosphere to fill with dust, casting an odd, reddish glow over much of the country.