The Belfast club announced the news late on Thursday night
Cork City left Belfast on Thursday night with a 1-0 win over Linfield, in the Europa League First Qualifying Round, but the result was marred by the death of a home fan during the game.
In a statement released on Linfield's website after the game, they announced the tragic news.
"The sympathy and condolences of everyone at Linfield FC are extended to the family and friends of the Linfield supporter who sadly passed away during tonight's Europa League game at Windsor Park.
Further details will be provided when they are known"
Sean Maguire scored the game's only goal, as Cork City take the one goal advantage back to Turner's Cross next Thursday night.
The death of the unnamed fan, if the third tragedy to fall upon Northern Irish fans in the past few weeks. Two fans, Darren Rodgers and Archie Rainey died in France when supporting Northern Ireland at Euro 2016.