Two crew members were injured when rival Celtic and Rangers fans clashed on a ferry to Northern Ireland.
Police in riot gear boarded the P&O crossing from Troon in Scotland when it docked in Larne, Co Antrim, last night.
Three men and a woman were arrested for assault and disorderly behaviour.
Around 100 of the 350 passengers on the ferry were supporters of the rival Glasgow clubs and involved in the trouble. Calm was restored about an hour later, and two members of the crew needed treatment for minor injuries.
Earlier Rangers and Celtic fans had clashed in the centre of Belfast. Police vehicles were damaged and an officer suffered minor injuries hours after the Old Firm game, won 2-1 by Celtic.
The football thugs went on the rampage after the Irish League was shamed by rioting at a showpiece match between Belfast's two biggest clubs.
Stewards were wounded amid the mayhem at Glentoran FC's Oval Stadium in the east of the city following a game against bitter rivals Linfield FC.
Fans who spilled on to the pitch fought hand-to-hand battles after the final whistle in what was effectively a league decider on Saturday.
Bottles and stones were thrown between the two sets of supporters in some of the worst scenes at an Irish League game for decades. Up to 400 fans were involved in fighting on the pitch after Saturday's game, won by Glentoran 3-2 with a last-minute goal.