Cathal Canavan, 24, was one of thousands of expats at Bronte Beach on December 25.
He is thought to have slipped on concrete while jumping from a cliff edge into the ocean pool in Sydney.
NSW Police, NSW Fire and Rescue and NSW Ambulance rushed to the packed beach.
Canavan was treated by paramedics before a specialist medical team was flown in by helicopter, which landed at Calga Reserve.
He was then rushed by ambulance to St Vincent’s Hospital.
The plumber’s aunt Audrey Guiney said he was in an induced coma, with his family being called on December 25 about the accident.
“They have him in a medically induced coma at the moment with brain damage but they don’t know the extent of the brain damage.
“They had to operate to save his life on Christmas Day but they think he’s out of danger now.”
His parents Shane and Josie and his brother Kieran have flown to Australia to be by his bed side, according to a fundraiser set up in Ireland.
It has gathered over $150,000.
Canavan, from Claremorris in Co Mayo, had reportedly only moved to Australia last year.
His sports teams from Ireland also posted online about the accident.
Claremorris GAA Club said on Facebook Cavanan had played his underage football up to Minor level.
“Our thoughts and prayers go out to him and his family,” it said.
Claremorris Hurling Club said the “young, energetic” member suffered a serious injury on Christmas Day.
“Our thoughts and prayers are with Cathal and his family,” it said.