Police in riot gear boarded a ferry in Larne after fighting broke out between rival football fans.
There was trouble on board a P&O ferry
Two crew members, one a woman, were slightly injured during disturbances on the boat after it left Troon on Sunday.
There were 350 passengers on board and it is thought up to 100 Celtic and Rangers fans were involved in fighting.
Police boarded the P&O ship at 2145 BST and arrested three men and a woman. They were later released on bail, pending further police inquries.
P&O marketing manager James Esler said the company was "horrified and disgusted".
"The primary concern is the safety of our passengers and crew - we are working with the PSNI, Celtic and Rangers football clubs and local supporters clubs in a review of what happened," he said.
Inspector Harry Miller said police had received reports of "considerable fighting and assaults" on the vessel.
"But both sets of supporters were calm and offered no resistance to police at all," he said.
The violence followed clashes between Celtic and Rangers fans in Belfast city centre on Sunday afternoon.
Celtic beat their Old Firm rivals 2-1 at Ibrox in a result which could secure the SPL title for the Parkhead club.
Police were attacked with stones and bottles after responding to reports of disturbances at the junction of Cromac Street and Ormeau Avenue at about 1530 BST.
Rival groups totalling about 60 youths were throwing missiles at each other.
One police officer received a minor leg injury and three police vehicles were damaged. A man and a woman were arrested.
Community representatives worked with police to end the disturbances.
Police believe youths from the Markets and Donegall Pass areas were involved in the disturbances, which lasted several hours.