Three people have been held as part of an investigation into the murder of Christopher Warren on 28 December, 2012.
The men were arrested in Dublin this morning during a search and arrest operation.
The three men, two in their 20s and one in his 60s, are being held at garda stations in the north inner city.
Mr Warren, 36, died after being shot in Phibsboro, Dublin.
A total of five people have now been arrested as part of the investigation into his death.