A man in his 20s has been arrested by detectives investigating the murder of Real IRA chief Alan Ryan.
The 32-year-old gang boss, who had links to extortion rackets in the capital, was murdered in Clongriffin, north Dublin, in broad daylight on September 3.
Two men have already been charged with withholding information in relation to the gun attack.
A garda spokesman said the latest arrest was made in the Artane area of north Dublin on Wednesday afternoon.
The man can be held at Coolock Garda Station for up to seven days under the tough anti-gangland law, Section 50 of the Criminal Justice Act.
"This is the fourth arrest as part of this investigation," gardai added.