Counterfeit goods worth €25,000 were seized during Manchester United's friendly fixture in Dublin last week, gardaí said.
Out of 21 vendors inspected near last Wednesday's game at the Aviva stadium, 17 were found to be selling fake produce, which can be used to fund organised crime groups.
Officers recovered approximately 225 hats, 200 placards, 140 flags and 2,220 scarves.
Detective Superintendent George Kyne said: "The ongoing co-operation between various elements of law enforcement will continue, the objective being to stop counterfeit and sub-standard products affecting the trade of genuine branded products.
"We will also continue to work with industry to ensure their intellectual property rights are protected in this regard."
United took on Italian side Sampdoria in a money-spinning warm-up to the Premier League season.
As part of a planned operation, police, customs officers, Dublin City Council trading licence inspectors and representatives from Manchester United and sports goods manufacturer Adidas carried out inspections near the ground in South Dublin.
Destruction orders will be sought from a district court for the fake products following the outcome of prosecutions.
A Garda statement said: "The selling of counterfeit goods such as the ones during this operation play a significant part in the funding of organised crime groups who operate throughout the country."