The three men were arrested at Dublin Port shortly before 8pm as they were about to board a ferry, while the woman was picked up later on the north of the city.
Carroll, a drug trafficker (33) was gunned down in a Coombe pub by a passenger on a motorbike that then sped from the scene.
Gardai are today questioning the four at Kevin Street, Pearse Street, Kilmainham, and Harcourt Terrace garda stations. The men are all aged in their 20s and the woman is aged 30.
Carroll owed more than €150,000 to drug-trafficking gangs, and a belief that he had been involved in a relationship with the partner of an an INLA activist may also be a factor in his death, which gardai believe was an opportunistic hit rather than a well-planned killing.
Garda intelligence indicates that he owed around €100,000 to one big drug trafficker from the Crumlin area, and that he had ignored several warnings to settle it. He also owed money to other dealers.
Carroll, a car sales manager, earned the nickname ‘Champagne’ because of his penchant for the high life.
He was shot four times in the stomach, arms and hips with a handgun as he sat drinking with friends in Grumpy Jacks pub in the Coombe area of the south inner city at around 9.30pm on Wednesday.
He had earlier been with friends watching soccer in a pub in Rialto and then Kilmainham, before agreeing to meet up with others in Grumpy Jacks.
Carroll had become a target for the garda national drugs unit because of his suspected involvement in a number of drug importations, and regularly changed allegiance with several gangs over the past few years.
Meanwhile, two brothers and a woman are still being questioned by gardai about the murders of drug trafficker Michael ‘Roly’ Cronin and associate James Maloney in the Summerhill area of Dublin on January 7.