Paul "Hippo" Ward, 34, who earned more than pounds 150,000 a year for collecting multi-million pound revenues from lower dealers, had denied the charge but was found guilty of being an accessory to the murder. His lawyers are to appeal.
The prosecution had earlier appeared on the point of collapse when Mr Justice Barr revealed that all three judges in the Special Criminal Court strongly doubted garda evidence that Ward had, after more than 14 hours of maintaining silence during interrogation, made "a remarkable volte- face" by suddenly admitted his role.
In the stunned aftermath of Ms Guerin's killing, the public laid wreaths in her honour outside the Dail, an eloquent protest at government inaction against a deteriorating crime situation. Ministers were embarrassed into response: an emergency Dail session passed new laws to seize assets from a rising generation of drugs millionaires.
The presumed invulnerability of the latter had been evident not just in conspicuous consumption but in their weapons arsenals, their incipient power wielded as a law unto itself. From 1994-1996, there had been 14 unsolved Dublin murders widely believed to be drugs-related contract killings.
Mother of a young son, formerly assistant to a party leader, a Dublin Fianna Fail treasurer and personally known to many senior politicians, Ms Guerin's special status had all too belatedly influenced events.
The eight-week trial was remarkable in three ways. Held in a no-jury court, the entire proceedings hinged on Charles Bowden, a former gang member turned state's witness, the subject of Ireland's first-ever witness- protection programme and now serving six years for drugs and arms offences.
It emerged that Bowden had been given "unconditional and irrevocable" immunity from prosecution over Ms Guerin's murder. Defence lawyers were refused access to 40 informants' statements. With further proceedings pending, the court ruled that nationally known criminals cited in testimony cannot be named.
Ward is the only man so far to be tried for the murder. The prosecution maintained that he helped plan it, that he disposed of the gun and motorcycle used in the attack, but did not take part in the shooting itself.
Ms Guerin died from bleeding and shock after being shot in the chest, back, and arm as her car pulled up at traffic lights in west Dublin. An hour later, Bowden said, one of the killers reported: "It was a good job."
In court, Ward, revealed how around a Caribbean swimming pool during a wedding trip to St Lucia in March 1996, gang members ridiculed their leader amid laughter after Ms Guerin brought assault charges against him. Bowden confirmed that the same man had been "pissed off" by the reporter's inquiries.
Garda James Hanley told the court that after his arrest, Paul Ward said he had only been asked to "look after the motorbike and gun used in the shooting". He said that asked to make a full statement, Ward had replied "Are you mad?... Charge me with taking the gun and the bike for them, but I won't take the murder rap." Ward's defence rubbished the statements as "contested verbals".
Yesterday, the judges agreed, concluding that Ward's alleged admissions, "if they took place... were induced by grievous psychological pressure". They suggested gardai had "terrified" Ward's elderly mother and kept his girlfriend, Vanessa Meehan, in unexplained custody. The judges also referred to allegedly "missing" garda documents and cited evidence of a possible assault by a garda on Ward because of his silence.
Ward, a heroin addict, had admitted earning pounds 300,000 from drugs between 1994 and 1996.
The prosecution rested on the credibility of Bowden, a former Irish army marksman, whom defence counsel Patrick MacEntee challenged forcefully. Bowden, 34, recalled being with Ward at a meeting of gang members which initiated Ms Guerin's killing, during a weekly gathering to divide cannabis profits. He revealed his share sometimes reached pounds 6,000 a week. When arrested he had pounds 85,000 in cash stored with a friend.
Bowden revealed that the gang had numerous imported weapons hidden in a Dublin Jewish cemetery. He had loaded the Magnum revolver used in the murder. Other evidence included records showing many mobile telephone calls between Ward's and the alleged assassin's numbers on the day of the murder.
Judge Barr said that Bowden, one of five "managers" within the gang, would lie regardless of the interest of others "if it was in his interests to do so".
Despite all this, the court ruled that Bowden's specific account of the killing was credible. Two alleged leaders of the drugs gang are facing court proceedings in Ireland and Britain.