A man and three women are being questioned by detectives in Cork in connection with a money-laundering investigation involving tens of thousands of euro.
All four are members of the same extended family with links to the drugs trade.
Three were arrested at two houses at Kerryhall Road in Fairhill in Cork shortly after 8am.
The fourth was arrested at an address on the Blarney Road in Cork.
All four are aged in their 20s and 30s.
They are being questioned at garda stations in Gurranebraher, Mayfield and the Bridewell in Cork.
They are being detained under section two of the Drugs Trafficking Act, which provides for detention for up to seven days.
Their arrests follow several seizures of drugs and cash by gardai from members of the family in recent months.
It is understood that gardaí have also been investigating bank accounts linked to the family, and transactions involving those accounts.