Three people are being questioned in connection with a threat to kill a journalist working for the Sunday World newspaper.
Reporter Mick McCaffrey was forced to leave his home for a time as a result of the threats.
Gardaí have released one of the men detained earlier while three others - two women and one man - are still being questioned in relation to the incident.
Last July, Mr McCaffrey, a crime reporter with the Sunday World, wrote an article about a criminal, who was on remand in prison for a shooting and other serious offences.
He then allegedly threatened to have the journalist killed.
Gardaí formally warned Mr McCaffrey who had to move out of his home.
Six people, including the criminal who made the threat, were arrested for questioning this week.
It is understood a threat has also been made against a detective in Kevin Street Garda Station.
The inmate behind the threat is part of a family-based criminal gang that includes two convicted killers, both of whom are in jail. One is serving a life sentence.