A Conor McGregor fan was pulled aside at Dublin Airport before boarding his flight to Las Vegas.
The MMA fanatic, who spent €2,500 (£2,200) on his trip in total, was left devastated after officials eventually told him to leave the airport.
The man had confessed to smoking Cannabis at the music festival Electric Picnic just weeks before his flight.
The fan’s Dad said: “He got through American immigration and he was sitting in the boarding area waiting for the flight and his pal to come through.
“He was pulled aside by an officer with a dog.
“The dog did not react to anybody but one of the senior officers told the other with the dog to pick out my son and another guy and pull them aside.
“They checked his passport and visa and everything was fine.
“The officer asked him had he ever been convicted of any drug-related offences and he said ‘no’ which is correct.
“Then the officer asked if he ever used any drugs. He said ‘no’.
“Then he asked if he had ever smoked any marijuana and my son said ‘well I tried it two weeks ago but that’s all and I never did it before, I don’t use it, I tried it at Electric Picnic because people around me were smoking it’.
“If it was a situation where that question was asked of a lot of people I’m sure they would answer the same.
“He wanted to be honest because we brought him up to be honest and tell the truth.”
The MMA fanatic’s Dad then said his son was taken into a room and made to wait with around 15 other people.
“He wasn’t spoken to anybody for around an hour, they came back, handed him his passport and walked him to the lift and said ‘passport control is that way you can now leave the airport’.
“They told him he wasn’t going to America and stamped his passport saying he was refused entry.”
The McGregor fan had answered “no” to the question about drugs on his visa form, which was correct at the time of application.
Officers said the form was not correctly filled out.
The fan’s dad said: “What they were saying was because he answered ‘no’ on that form was that he should have updated that form.
“We went into his account yesterday after he got home and tried to update the form and there was no way of doing it.
“He was so looking forward to this, he’s followed MMA for a long time, he’s worked hard to get the money together, and he’s devastated.”