A police search at Gaelic Athletic Association grounds in west Belfast is understood to be linked to last December's £26m Northern Bank robbery.
Millions were taken from the bank's vaults last December
The GAA expressed shock at the investigation at Casement Park, which a police spokesman said was part of an investigation into serious crime.
Chairman of the Antrim County Board Joe O'Boyle said Casement Park Social Club had already co-operated with police.
He said a man arrested on Tuesday, Chris Ward, worked there part-time.
On Thursday, police were given another three days to question Mr Ward, who has previously said his family was held captive during the raid.
Mr O'Boyle said: "I got information that 25 Land Rovers arrived with search warrants to do a search of Casement Park social club.
"Chris Ward is a part-time worker at the social club.
"The PSNI visited Casement Park approximately nine months ago and we given all the information they requested so I don't know why they have returned."
The GAA said in a statement it had no prior knowledge of the operation and was not given any reason for the inquiry, other than an indication it related to allegations about an individual and alleged activities unrelated to the GAA.
The GAA said it had reported the matter to the Dublin government.
Another man arrested by police investigating last December's robbery has been released without charge.
The 50-year-old, from Carrickfergus, County Antrim, was arrested on Friday under the Terrorism Act.
A 22-year-old woman who was arrested on Tuesday was released without charge a day later.
Meanwhile, the lawyer for a man charged in connection with the robbery told the High Court in Belfast on Friday his client had been prevented from making a bail application.
The lawyer said the application, on behalf of Dominic James McEvoy, could not be made because the authorities were "withholding vital information".
Mr McEvoy, 23, from Kilcoo, County Down, was charged after his DNA was allegedly found at the Loughinisland home of a bank employee who had been held hostage.
He was remanded in custody until 9 December.
Earlier this month, police investigating the robbery arrested several people.
Of the 11 people questioned to date in connection with the robbery, three have appeared in court.
The robbery happened at the bank's Northern Ireland headquarters at Donegall Square West just before Christmas last year.
Some money seized in County Cork last February was linked to the robbery, but virtually all of the missing millions remain unrecovered.
The Chief Constable, Sir Hugh Orde, subsequently blamed the IRA for the raid.