Tributes have been paid to Dr John Hinds, one of the "flying doctors" of Irish road racing.
He was involved in an accident while providing medical cover at a Skerries 100 practice session, the Motorcycle Union of Ireland said.
He died in hospital in Dublin on Saturday.
Dr Hinds, 35, from Tandragee, and his colleague Dr Fred McSorley were dubbed "the flying doctors" of Irish motorcycle sport.
He was a consultant at Craigavon Area Hospital in County Armagh.
He regularly worked at the North West 200 motorbike races in County Antrim.
Dr McSorley told BBC NI's Good Morning Ulster on Monday that an investigation into the cause of the accident was under way.
"John was not racing, he was riding his road bike. He was not in a race, we do not race," he said.
"He was travelling with the marshals after the race. He had been on the circuit that day. It was not a tricky corner. He was a highly skilled rider and we really don't understand what happened," he said.
Paying tribute to Dr Hinds, he said his death was
"profoundly difficult for everyone".
"In road racing, he was revered and adored by so many riders.
"But our thoughts also go to staff in Craigavon where he was revered for his wonderful work. He was a natural teacher and wonderfully inspirational.
"He was hugely dedicated to improving the care of critically ill people."
Dr McSorley said Dr Hinds was exceptional.
"He not only saved people's lives but effectively, particularly with head injuries, he saved their brains.
"The fact they are able to walk and talk normally, get to work, pick up their children and give them a hug is because of the excellent care that he provided."
Harris Healey from the Motorcycling Union said Dr Hinds was
"always a gentleman".
"He saved a lot of people's lives in racing accidents and people had a better outcome because he was there," he said.
Dr Hinds had led a campaign for an air ambulance service to be introduced in Northern Ireland and had met the health minister,
Simon Hamilton, to discuss it.
He said Dr Hinds was a
"lovely guy who will be sadly missed".
TUV MLA Jim Allister, who backed the air ambulance campaign, said the death of Dr Hinds was deeply shocking.
"He was one of our leading trauma experts. He was driven with a great passion to try and improve the chances of those involved in serious accidents. My thoughts are with his family.
"I do hope as a lasting tribute to him, the government will now push ahead with his ambition to have a proper air ambulance service for Northern Ireland."
Members of the medical profession have also paid tribute to the Tandragee man.
The Northern Ireland Ambulance Service (NIAS) tweeted:
"Saddened to hear reports of the untimely death of Dr John Hinds - a friend to many in NIAS."
Members of London's Air Ambulance service said Mr Hinds "was a true friend and advocate of the air ambulance community".