The billionaire has refused to leave his retreat, in the British Virgin Islands, as the terrifying winds and torrential rain head straight towards it.
Instead he and his team will hide in the wine cellar at his luxury estate when the full force of the Category 5 storm hits.
'We are expecting to get the full force of the hurricane in around five hours' time, when we will retreat to a concrete wine cellar under the Great House," Branson wrote on his blog.
"Knowing our wonderful team as I do, I suspect there will be little wine left in the cellar when we all emerge."
Richard Branson is refusing to leave Necker Island as Hurricane Irma approaches
The business mogul added that everyone at the estate slept in just two rooms last night as they prepared for the extreme weather.
"The strength of this hurricane is unheard of," Branson said.
"The atmosphere is eerie but beautiful. Everyone is willing the eye of the storm to veer away from the BVI in these last few hours.
"Our main concern is safety, for everyone here and for all the people in the BVI and in the path of the hurricane.
"All of us slept together in two rooms. I haven't had a sleepover quite like it since I was a kid.
"Strangely, it's a privilege to experience what is turning into possibly the strongest storm ever with such a great group of young people.
Yesterday Branson shared photos of how he was preparing for the hurricane
"We were listening to the parrots in their boxes in the next room chattering away. Watching the tortoises congregating together, as if they sense what is coming our way."
Yesterday the Virgin boss described how he had survived two hurricanes on the island thanks to his specialised 'hurricane proof' buildings - but he said "nothing can withstand" a Category 5 storm.
Branson wrote: "Generally speaking, we have one hurricane in the British Virgin Islands around every 10 years.
"Fortunately, most of them drift north of us, but this one is coming straight for us, with the eye of the storm heading straight for Necker, Moskito Island and Virgin Gorda.
The billionaire admits that "nothing can withstand" a category 5 hurricane
A satellite image of Hurricane Irma taken yesterday
Necker Island in the British Virgin Islands, where Branson's luxury retreat sits
"Obviously things can change – hopefully they do, as a category five hurricane hasn't hit the BVI [British Virgin Islands] full on before.
"On Necker Island we have constructed really strong buildings (with hurricane blinds) that should be able to handle extreme weather pretty well, though with a Category 5 hurricane almost nothing can withstand it.
"We had some lovely guests staying on Necker Island who have cut their trip short for safety reasons, and another group of guests have also postponed.
"I will be on Necker alongside our team, as I have been on the three times we have had hurricanes over the past 30 years."
Branson, who said he was on the island in 2010 when it was hit by Hurricanes Earl and Otto, went on to share his concerns for local residents and wildlife.
Hurricane Irma is barrelling towards Florida
He added: "In the past, many British Virgin Islanders have shrugged off hurricanes, stayed at home and not gone to shelters.
"This time the BVI Government and the BVI Tourist board have been very active and providing advice to the local population."
Irma is set to hit Necker Island less than two weeks after Hurricane Harvey killed 60 people when it battered the United States and surrounding territories.
Hurricane Irma reached the Leeward Islands, east of Puerto Rico, this morning and will likely batter Florida by the weekend.
People buying materials to protect their homes after Puerto Rico Governor Ricardo Rossello declared a state of emergency yesterday
Two women prepare for the storm heading to Marigot, on the French overseas island of Saint-Martin
Florida has declared an emergency and President Donald Trump has "offered the full resources of the federal government as Floridians prepare for Hurricane Irma".
"In Florida, we always prepare for the worst and hope for the best, and while the exact path of Irma is not absolutely known at this time, we cannot afford to not be prepared," said Governor Rick Scott in a statement.
Water is expected to rise 9ft (3metres) above normal levels as the storm brings a predicted 10 inches of rain and "large and destructive waves".
Locals are taking no chances
Irma was initially a Category 4 hurricane on the five-step Saffir-Simpson scale, meaning sustained winds of 130-156 miles per hour (209-251 kph) with "catastrophic" outcomes.
This includes the uprooting of trees and downed power lines, water and electricity outages, and significant property damage causing uninhabitable conditions, according to the National Hurricane Center.
The NHC said: "There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend.
"In addition, rough surf and dangerous marine conditions will begin to affect the southeastern U.S. coast by later this week."