The 28-year-old complained of feeling unwell after Saturday's fight against Irishman Charlie Ward was stopped by the referee in the third round.
The Portuguese welterweight, who fought for Team Nobrega, was taken to hospital and had emergency brain surgery that night but died on Monday evening after 48 hours in intensive care.
A statement from Total Extreme Fighting
(TEF), which staged the event at Dublin's National Stadium, confirmed Carvalho's death.
"We extend our most sincere condolences to the family of Joao Carvalho, and to his teammates in Team Nobrega," TEF chief executive Cesar Silva said.
"Our thoughts and prayers are with them. We'd also like to thank all sports fans for their concern and support."
The statement said TEF would "give whatever support we can to Joao's family" and noted that the fighter had been admired on both "a national and international level."
, where Carvalho had been all his career, said it was "saddened and dismayed" about the death.
It stressed that "all safety regulations were observed" in Dublin and said Carvalho had become ill around 20 minutes after the end of the fight.
He "was immediately helped by the medical team" at the stadium before being transferred to hospital.
"Although the risks of this sport are known, the death of Joao Carvalho was a huge tragedy," team chief Vitor Nobrega said.
Irish UFC champion Conor McGregor had previously said he felt the fight "could have been stopped a little earlier."
John Kavanagh, who coaches McGregor and was in Ward's corner, issued a statement in which he said: "It is with heavy hearts that we mourn the passing of Portuguese fighter Joao Carvalho.
"Our condolences and thoughts, though, are with Joao's family and friends and we kindly ask everyone to respect their privacy at this time."
Kavanagh said the Irish Amateur Pankration Association, which represents MMA in the Republic of Ireland, would work with Total Extreme Fighting "to gather and evaluate all relevant facts and participate in any investigation."
The popularity of MMA, in which fights are held in octagon cages, has increased in Ireland since McGregor's rise to prominence.
Bouts in Dublin regularly attract thousands of fans, while media coverage of the sport has grown significantly.