Gardaí fear an escalation of the feud between the Ryan and Keane families in Limerick following the shooting dead last night of Mr John Ryan.
A murder investigation was under way following the drive-by shooting in Thomandgate, in the north of Limerick city at 6.30 p.m.
Two men were being questioned last night in connection with the shooting. They were arrested in the Thomandgate area and were being held at Mayorstone Garda Station.
Late last night there were reports of a further shooting in Garryowen and of a mini-riot in the Island Field area of the city.
Mr Ryan was the brother of Mr Eddie Ryan - who was murdered in the Moose Bar, Limerick, in November 2000, and an uncle of Eddie and Kieran Ryan whose alleged abduction in January led to an escalation of the feud.
He was shot a number of times in the back and leg outside a house in Canon Breen Park, Thomondgate. A number of previous attempts had been made on his life.
According to witnesses, Mr Ryan, who was in his late forties, was working on a patio in a front garden when two men pulled up on a scooter. At least four shots were fired by the pillion passenger, who was armed with a handgun. Mr Ryan was taken by ambulance to the Mid-Western Regional Hospital in Limerick where he was pronounced dead shortly after 7 p.m.
A witness said: "I saw the man lying on the ground. There was blood everywhere." Detectives were quickly at the scene and several houses were cordoned off while forensic tests were carried out.
Gardaí were investigating reports last night that shots were fired at a house in O'Malley Park on the southside of the city minutes earlier. One theory was that the first shooting incident may have been a decoy to distract mobile Garda patrols.
Gardaí fear the killing will spark a further escalation in the vicious ongoing feud which is believed to have led to the murder of Mr Kieran Keane in January this year and to more than 100 other "serious" incidents.
One man is currently on trial before the Central Criminal Court for the role he allegedly played in Mr Eddie Ryan's murder.
Earlier this year, the late Mr Eddie Ryan's sons - Kieran and Eddie jnr - were the victims of an alleged kidnap which led to a massive Garda manhunt.
However, they turned up safe at a Garda station in Portlaoise a week later. Just a few hours after that, Mr Kieran Keane (36) was found murdered in Drombana on the outskirts of Limerick city. He had been shot in the head. His nephew, Mr Owen Treacy, survived multiple stab wounds in the same incident.
Mr Keane was one of the chief suspects in the murder of Mr Eddie Ryan but was not charged in connection with his killing.
Five men are currently before the courts charged with Mr Keane's murder and with the attempted murder of Mr Treacy.