Gregg Bemis, 83, who has owned the wreck since 1968, oversaw operations off the south coast on Saturday as divers began cutting through the hull of the wreck.
He is funding the multi-million dollar expedition, based on the Irish Lights vessel, the Granuaile, which is anchored some 90 metres over the wreck site about 25 nautical miles off the Old Head of Kinsale.
Wearing hi-tech atmospheric diving suits developed by Nuytco, which allow divers remain submerged for three-hour shifts, the divers used specialised cutting torches to breach the hull.
Weather permitting, they hope to remove the cut section within the next few days.
They will then send ROV (remotely operated vehicle) submarines inside the wreck to hunt for tell-tale signs which could explain why the luxury Cunard liner sank so fast.
“What we are hoping to do is basically cut open the side of the Lusitania and send a remotely operated vehicle inside the vessel to explore the interior compartments and hopefully find out why she sank in less than 20 minutes after the torpedo hit,” said Aidan Mulcahy of Cork-based M3TV, which is filming the expedition for National Geographic.
If they solve the riddle, it would bring to an end almost a century of speculation about the 1915 tragedy.
The expedition team held a memorial service for the victims of the disaster before work started last week.
Mr Mulcahy also said the work, which is being conducted under special licence, is being overseen by conservation experts.
“We wanted to acknowledge the tragedy of what happened and to do this work in a respectful way,” he said. “It’s only when you’re out here, and realise how close you are to land, that you realise that this is where hundreds upon hundreds of people had their last moment of life.”
The Lusitania, which was known as ‘The Greyhound of the Sea’, held the speed record for crossing the Atlantic until 1909, when she lost it to her sister ship, the Mauretania.
It was 25 nautical miles south of the Old Head of Kinsale en route from New York to Liverpool in May 1915 when it was hit under its bridge by a torpedo fired from a German U-boat.
The explosion triggered a mystery secondary explosion which ripped the hull of the 790ft (241m) vessel apart.
It sank by the head in less than 18 minutes, killing 1,198 of the 1,959 people on board, including 39 children and dozens of Americans.
The sinking caused massive controversy because the vessel was carrying civilian passengers, including eminent and wealthy politicians, artists, the art collector Hugh Lane, academics and businessmen.
The disaster also increased the pressure on the US to enter World War I.
There has been speculation since about what caused the second explosion.
The U-boat captain was accused of lying about how many torpedoes he fired.
In 1993, Robert Ballard, who discovered the wreck of the Titanic, said he believed dust in the ship’s coal bunkers was thrown into the air by the torpedo impact and ignited, causing the second explosion.
But Bemis has always claimed the ship’s secret ammunitions cargo caused the second explosion.
“Since the beginning of my involvement, over 40 years ago, I have wanted to find out what caused the second explosion,” Mr Bemis said. “The ship is lying on its ruptured starboard side, so there is no easy way of doing it and examining the area the torpedo went in. That area is totally concealed. We have no way of digging down. So what we hope to do is go through the port side and find evidence of what caused the explosion there.”
Five different cameras are filming the wreck and the expedition for the two-hour documentary, The Dark Secret of the Lusitania, which will be broadcast on National Geographic in 2012.