Two of the five, a man in his fifties and a woman, were held after they left a ferry from Holyhead, Anglesey, when it docked at Dun Laoghaire, Co Dublin, yesterday morning.
The move was understood to have followed a stake-out operation by detectives at Irish airport and ferry terminals.
Three other men were held later in other parts of the Irish capital. All of the arrests were made under legislation permitting suspects to be held in custody for up to 48 hours before being either charged or released.
The man held on the ferry had been sought by police for questioning since leaving Ireland for Britain at the end of last summer.
Police have questioned over 100 people since Ms Guerin was shot dead at the wheel of her car in Dublin last June.
The on-going investigation has also produced scores of underworld guns and other weapons, and led to prosecutions not directly linked to the killing of the journalist.
So far, one person - a Dublin man - has been accused of the Guerin murder.
The shooting prompted widespread outrage in the Irish Republic and led the Dublin parliament to introduce a series of anti-crime measures - including the right to seize the assets of known criminals.Reuse content