Twenty-six-year-old Paul Kelly was shot six times as he opened the door of his sister's apartment in Clontarf, Dublin, on Friday night. Kelly, from Gardiner Place, was regarded by gardai as a petty criminal who specialised in stealing high-powered cars and motorbikes. He had a small number of convictions for theft and other minor offences and fell out with a local drug dealer six months ago over a woman.

There have been several minor incidents of threats and violence since then but the feud escalated last week when Kelly was accused of stabbing a relative of the drug dealer near Summerhill.

The man was said to be carrying a young child when he was stabbed three times with a knife, receiving minor injuries. Gardai never received a complaint about the incident but were told the drug dealer swore revenge.

The dealer is in his mid-20s and is on bail after being charged with the possession of a large amount of cocaine last year. The dispute was so minor that gardai didn't even know about it until recent weeks.

Paulie Kelly was in his sister's apartment at Winstonville on Charlemont Road when he was targeted by a masked gunman just before 11.50pm. He was shot six times in the head and body and died instantly.

The assassin made his escape in a waiting vehicle and gardai were on the scene within minutes. They received intelligence suggesting a named person had been seen near the murder scene and during follow-up searches four men and a woman were arrested at two addresses in the north inner city.

Paulie Kelly was also the prime suspect in a gun attack that left a teenager from Rutland Street with serious injuries in 2004. He was arrested as part of the garda investigation but could not be charged because the victim withdrew his complaint.

One garda said: "Paulie Kelly was a messer more than anything else. He was an expert car and bike thief. He recently fell out with a dealer from Summerhill and there have been a few incidents over the past month but nothing to suggest he was going to be murdered. He wasn't on our radar at all but it just shows that a petty squabble can quickly turn into a gun attack. The problem with feuds like this is that incidents rarely get reported to us. We only heard about this supposed stabbing last week from local sources."

A massive garda presence has been maintained around the Sheriff Street area over the last six months as a result of a feud between two local families that has left two people dead so far.

Gerard 'Batt' Byrne and Stephen Ledden were murdered in a row over the rape of a 13-year old by a member of the IRA in 1998.