A 51-year-old criminal from Dublin's northside is one of two men aged in their 50s being questioned last night about the Regency Hotel murder.
The suspect, who is from the East Wall area, was arrested on Monday evening and is being held at Clontarf Garda Station.
Armed gardai arrested a second suspect yesterday morning and he is being detained at Ballymun Garda Station.
A garda spokesman said that a total of seven people have now been arrested for the Regency Hotel murder.
One of these, a 24-year-old Dublin man was charged last month with David Byrne's murder on February 5.
The 51-year-old criminal who remained in garda custody last night has served jail sentences for offences including a savage assault and possession of stolen property.
He also has numerous convictions for driving offences.
Sources said that detectives don't believe that either of the men arrested this week were the gunman in the Regency Hotel attack during which David Byrne (33) was shot dead.
Gardai suspect over a dozen people were directly involved in the shooting and subsequent getaway effort.
Five gunmen stormed the hotel - three dressed as fake garda ERU members and carrying AK47s - during the attack.
Another individual drove the Ford Transit van from the scene, while a number of cars, including taxis, were used to collect the hit team from the Charlemont housing estate, where the van was later found burnt out.
Gardai also believe a gang member acted as a spotter for the hit team.
Officers have not yet arrested a dissident Republican suspected to have been involved in the gun attack.
It is understood that gardai are preparing a file for the Director of Public Prosecutions.
Regency Hotel murder victim David Byrne was from Raleigh Square in Crumlin and had been a huge target for specialist garda units for years. He had been a key Dublin-based member of the Christy Kinahan cartel for years.
A younger brother of feared criminal Liam Byrne, David was cleared of firearms charges in relation to a shooting incident connected to the Crumlin/Drimnagh feud which occurred in August, 2008.
Gardai have deployed patrols across both sides of the city in a bid to quell the feud.