The storm has sustained winds reaching almost 300kph
The most powerful Atlantic Ocean hurricane ever recorded is causing havoc on several Caribbean islands, with some area braced for winds of almost 300kph.
The eye of Hurricane Irma first roared over Barbuda at 1.47am local time, also hitting the nearby island of Antigua.
St Barts and St Martin were next to be hit, as low-lying areas flooded, according to the French weather office.
All government buildings on the French-run side of the island of St Martin had been destroyed, according to French interior minister Gerard Collomb.
"We know that the four most solid buildings on the island have been destroyed which means that more rustic structures have probably been completely or partially destroyed," he told reporters.
French overseas territories minister Annick Girardin said Irma had caused "major damage" on several islands.
The hurricane has maintained category five strength with sustained winds near 185mph (295kph), the US National Hurricane Centre said.
According to some weather experts, the hurricane covers an area bigger than the UK and Ireland.
The weather station on St Barts measured winds of 151mph (244kph) before its monitoring equipment was destroyed by the storm.
"We are hunkered down and it is very windy... So far, some roofs have been blown off," said Garfield Burford from Antigua and Barbuda's ABS TV and Radio.
Most people were without power and around 1,000 people on Antigua were spending the night in shelters, Mr Burford said.
"It's very scary... most of the islands are dark, so it's very, very frightening," he added.
The wind sent debris flying as people huddled in their homes, many of which do not have concrete foundations and only wooden roofs.
Tourists in the area were evacuated on Tuesday before the hurricane arrived.
The Department of Foreign Affairs says Irish should follow the advice of the local authorities and any orders given to evacuate to or remain in a place of safety.
It also says people in affected areas should monitor local weather updates regularly.
Irma is currently heading through the Caribbean towards Florida and may make landfall at the weekend, but its precise path is uncertain.
The most dangerous winds are forecast to pass near the northern Virgin Islands and near or just north of Puerto Rico during the day on Wednesday.
Hurricane warnings are in place on several islands, including the British Overseas Territory of Anguilla, Montserrat, St Kitts and Nevis, the US and British Virgin Islands, Puerto Rico and the Dominican Republic.
President Trump has declared emergencies in Florida, Puerto Rico and the US Virgin Islands.
If it hits Florida it will be the second powerful storm to hit the US mainland in two weeks after Hurricane Harvey caused devastation and led to the deaths of 60 people.
Bahamas Prime Minister Hubert Minnis has also said the government there will evacuate six southern islands as authorities would not be able to help people caught in "potentially catastrophic" conditions.