Conor McGregor has described Portuguese fighter João Carvalho, who died following an mixed martial arts (MMA) event in Dublin on Saturday, as a “hell of a fighter and will be sorely missed by all”.

The international governing body of MMA and its Irish affiliate have said they are investigating the circumstances surrounding the death of Carvalho, who was taken to Beaumont Hospital following a bout organised by Total Extreme Fighting (TEF) against Irish fighter Charlie Ward at the National Stadium.

McGregor, the Ultimate Fighting Championship (UFC) featherweight champion, issued a statement on Tuesday evening calling the death of the 28-year-old “truly heartbreaking”.

“We are just men and women doing something we love in the hope of a better life for ourselves and our families. Nobody involved in combat sports of any kind wants to see this. It is such a rare occurrence that I don’t know how to take this. I was ringside supporting my teammate, and the fight was so back and forth, that I just can’t understand it.

“My condolences go out to João’s family and his team. Their man was a hell of a fighter and will be sorely missed by all. Combat sport is a crazy game and with the recent incident in boxing and now this in MMA, it is a sad time to be a fighter and a fight fan.

“It is easy for those on the outside to criticise our way of living, but for the millions of people around the world who have had their lives, their health, their fitness and their mental strength all changed for the better through combat, this is truly a bitter pill to swallow. We have lost one of us.

“I hope we remember Joao as a champion, who pursued his dream doing what he loved, and show him the eternal respect and admiration he deserves. Rest in peace, João.”

The contest was stopped by the referee in the third round due to a technical knockout, which is when a fighter is deemed by the referee to be unable to intelligently defend themselves or safely continue the fight.

Carvalho was checked by doctors at the stadium but later complained of feeling unwell and began vomiting. He was taken to hospital where he underwent emergency brain surgery, but he died from his injuries on Monday.

Trainer Vitor Nóbrega said the event “complied with all the safety rules” and that arbitrators had followed “all the correct procedures”. He added Carvalho was “immediately assisted” by the medical team and “quickly” transferred to Beaumont.

TEF chief executive Cesar Silva said medical reports were still being compiled by the hospital but that “normal procedure” was followed.

“Following the referee stoppage, João, who was representing Portuguese club Team Nobrega, was assessed by doctors and medical staff at the onsite medical office as per normal procedure,” he said.

“It was determined that he should be sent to hospital. In hospital he received emergency treatment and transferred to ICU later that evening. Formal hospital medical reports and procedures are still being carried out.”

A spokesman for the Garda said the matter was being treated as a “sudden death incident” and that a file was being prepared for the coroner’s court.

Minister for Sport Michael Ring said he expects an investigation to take place into the circumstances surrounding the incident and would not be commenting further until its results were known.

John Treacy, chief executive of Sport Ireland, which is the national agency for sport in the Republic, said safety regulations were the responsibility of sports’ individual governing bodies.

“We haven’t been approached formally [by MMA authorities] for recognition,” he said. “It’s up to sports individual governing bodies to ensure they have the proper safety regulations in place. That’s part of their responsibility.”

The international democratic governing body for MMA is the International Mixed Martial Arts Federation (IMMAF). Its president Kerrith Brown said the organisation was in the process of “gathering information”.

“We are in the process of gathering information with the support of the Irish Amateur Pankration Association (IAPA) and await the outcome of official medical reports,” he said.

The IAPA is officially affiliated with the IMMAF and was formed in late 2014 with the stated purpose of “creating a framework where Irish MMA would become recognised as a legitimate sport in Ireland”.

Its president, John Kavanagh, who is also the coach of McGregor, said the organisation was "working diligently" to gather all the facts.

“IAPA is working diligently with TEF to gather and evaluate all relevant facts and event processes and will participate in any investigation,” he said.

Charlie Ward’s father, Charles Ward, told Liveline on RTÉ Radio 1 on Tuesday that his family was struggling to cope with the news, and that his son said to him after the fight: “No way were those punches hard. No way was I hitting him hard.”

Mr Ward said the referee could have stopped the fight earlier. “He didn’t go out to kill anyone,” he said. “He went out to win his fight, but the referee could have been in a bit quicker.”

Medical care for the event was provided by private health company Eventmed. According to its owner, Katarzyna Michlic, full post-fight medical protocol was observed.

“He was asked if he felt any pain or was suffering with any headaches directly after the fight, and he said he had no pain or headaches, but he did feel quite tired,” said Mr Michlic.

“It was explained to him that he had to visit the medical room that was set up in the venue and that he would have to go to the hospital afterwards, because the doctor recommended that he should have a CT scan after the fight.

“At this stage, the fighter was walking, talking and smiling. Everything seemed perfectly fine. After he walked back to the medical room the doctors did some tests backstage and there was nothing out of the ordinary about their results. They prepared the ambulance to take him to the hospital.

“Ten minutes later he started complaining that he was suffering from headaches. He felt a lot of nausea and he began to vomit. Five minutes later he was significantly worse so we got him into the ambulance and rang the hospital to let them know he was coming.”

A spokesman for the National Stadium said it was “deeply saddened” to learn of Carvalho’s death and extended its “deepest condolences” to his family.