Gardaí investigating the murder of Mark Desmond have released four of the five arrested people, without charge.
A file will now be prepared for the DPP.
The three men and one woman were arrested yesterday evening and late last night.
One man, aged in his late twenties, remains in Garda custody.
Gardaí said they are continuing their investigation.
Desmond, who was known as the 'Guinea Pig', was shot dead last December in a west Dublin park. He was the chief suspect in at least four murders and had been working for a drugs gang at the time of his killing.