Aer Rianta is conducting an internal investigation after one of its security staff allegedly asked a Northern passenger at Dublin airport to remove his Rangers shirt.
The passenger complained that other people around him were wearing Celtic tops without complaint from Aer Rianta.
The row began when a Northern passenger at Dublin airport complained he and friends were asked to remove their Rangers shirts.
He said they either took the shirts off or covered them up with jackets as they were worried about missing their plane.
But he is alleging discrimination because others around were wearing Celtic tops without any problem.
He says the explanation given by the Aer Rianta security man was that there are more Celtic fans in the Republic.
Vincent Wall, director of communications at Aer Rianta, said if such an incident took place it was inappropriate and the company was investigating to see exactly what had happened.
If it transpired that anyone was told to take off a football shirt, severe disciplinary action would be taken, he said.