Brian O'Driscoll orchestrated a masterful victory for Leinster at Franklin's Gardens that has done significant harm to Northampton's hopes of progressing in the Heineken Cup.
The vision of man of the match O'Driscoll created the opening two touch downs for Fitzgerald in a humiliating evening for Saints that saw them concede the bonus point as early as the 37th minute.
Jamie Heaslip and Eoin Reddan also crossed in the first half as Leinster issued a statement of intent in their quest to claim a fourth Heineken Cup crown.
They led 26-0 at half-time in the first meeting between the teams since the 2011 final and the rout continued after the interval with O'Driscoll, playing his first Heineken Cup match of the season, and Fitzgerald adding further tries.
Fly-half Ian Madigan slotted five conversions against the Aviva Premiership's second placed team.
Saints were struck by a setback in the warm-up when full-back James Wilson withdrew injured, forcing wing Ken Pisi to fill the void.
It proved an unfortunate turn of events as mistakes made by Pisi contributed to two of Leinster's early tries as the 2009, 2011 and 2012 champions made a blistering start.
The 33-point margin of defeat equalled Northampton's previous greatest defeat in Europe, against Clermont in 2001-02.
Leinster's dream start began with Fitzgerald running in two tries created by O'Driscoll inside the opening eight minutes to establish a 12-0 lead.
The first arrived after Pisi dropped the kick off, giving the Amlin Challenge Cup champions a scrum from which O'Driscoll produced a grubber.
The ball bounced kindly for Fitzgerald and with Rob Kearney impeding Pisi, the Ireland wing was able to touch down with just 150 seconds on the clock.
Better was to come from O'Driscoll in the eighth minute when his flicked a pass through his legs to Kearney, who sent Fitzgerald side-stepping over.
Rattled Saints were awful as North missed a simple catch and Pisi knocked on in a promising position, the prelude to leaking two more tries in the second quarter.
Sean O'Brien bulldozed his way upfield and when the ball was sent right, Kearney created space with the help of some weak defending for Heaslip to stroll over.
The try-scoring bonus point was secured with four minutes of the first half remaining and the blundering Pisi was culpable once again as a high ball was snatched from his grasp by Kearney.
Several drives later and Reddan exploited a large gap around the breakdown and side-stepped Jamie Elliott to dart over.
Leinster would have breached the whitewash for a fifth time had a pass from hooker Sean Cronin, who brilliantly slipped through Saints' midfield, had not travelled forward to Fitzgerald.
O'Driscoll and Fitzgerald had done the hard work but Reddan juggled and lost the ball over the line as another score went begging.
Finally the fifth try came as desperate Northampton sought to break from deep inside their own half, only for O'Driscoll to intercept a pass by Dom Waldouck and race over.
England scrum-half Lee Dickson pumped his first as he barged over from close range, but Leinster replied with their sixth try when Fitzgerald tracked Zane Kirchner and took an easy scoring pass to complete the rout.