The Total Extreme Fighting 1 card took place at the National Stadium in Dublin on Saturday night.
One of the bouts on the card was between Irish fighter Charlie Ward and Joao Carvalho who is Portuguese.
Ward won the fight by TKO in the third round.
Initially, the Portuguese fighter appeared in OK condition after the fight. However, according to the Facebook page of his Gym - Nobrega Team - 20 minutes later, Carvalho began to feel ill and was taken to hospital.
Today's Irish Daily Star reports that Carvalho underwent emergency surgery at Beaumont hospital and is currently in critical condition.
The paper also quotes CEO of Total Extreme Fighting Cesar Silva.
Joas's welfare is our primary focus at this moment in time. Last night's [Saturday] fight was SAFE MMA approved, with three doctors and paramedic staff present.
Their presence adhering to SAFE MMA regulations ensured that the seriousness of Joao's condition was recognised and swift action taken and we are thankful for their help.
Serious injuries like this are rare worldwide in MMA but it is important that every possible precaution is taken to ensure every competitor's welfare.
Charlie Ward fights out of SBG - the same gym as Conor McGregor - under the tutelage of John Kavanagh.