The GAA has cancelled some tickets for the All-Ireland football semi-final between Tyrone and Dublin that were being sold online at above face value.
It has warned fans turning up for the match with cancelled tickets that they will be refused entry at the turnstiles.
Tickets with a face value of 45 euros (£40) are fetching up to 190 euros on the black market.
A GAA spokesman said more than 10 tickets had been cancelled in 24 hours.
"To the people that have paid over the odds, they face a real risk arriving at Croke Park, into the Hogan Stand, or Cusack Stand, that will not gain them admittance," GAA Director of Communications Alan Milton told RTE.
"They are taking a huge risk, both financially and also in gaining admittance to the stadium."
He stressed the importance of purchasing tickets from official sources.
The Dublin-Tyrone game on 27 August is heading towards an 82,300 Croke Park sell-out, but holders of tickets from unofficial sources may find their unique barcode will not scan.
Mr Milton said it was wrong to profit from fans attending music and sporting events, and that genuine fans had been contacting the GAA aggrieved that they were unable to afford the black market prices.
He said the GAA regularly checked websites and that it had even cancelled executive boxes in an attempt to stamp out the practice.
"I see it as being fundamentally wrong," he said. "It's unjust."