The 24-year-old - who was named man-of-the-match - rounded off a much-improved second-half display from the home side by crossing for one of four scores after the interval.
The Irish province had trailed 7-0 at the break following Nick Williams' score, converted by Tito Tebaldi.
However tries from Ian Madigan, Isa Nacewa, Fitzgerald and Rhys Ruddock saw Leinster home as they sealed a third win from their opening five games.
After the usually dead-eyed Fijian Nacewa missed three penalties early in the first half Aironi No. 8 Williams gave the Italians a deserved 35th-minute lead.
With Aironi having dominating territory for large parts of the half Williams found a way over the line with Italian international scrum-half Tebaldi adding the extras.
After gaining their first victory of the season against Edinburgh last time out, Rowland Phillips' side were looking like making it successive victories, despite going down to 14 midway through the first half after Samoan winger Sinoti Sinoti was sin-binned.
Leinster, who ran out 15-10 winners away at Scarlets last Saturday, came out a different side in the second half and responded through Madigan who benefited from good work by his forwards. This time Nacewa found his range and kicked the conversion to level the scores.
Fly-half Nacewa then scored a well taken try of his own on 58 minutes seemingly putting his poor kicking display behind him, but missed the conversion.
Ireland international Fitzgerald then put breathing space between the sides 12 minutes from time after he intercepted a badly-timed pass from Tyson Keates to run uninterrupted to score Leinster's third try of the game.
And captain Rhys Ruddock's last minute try ended the contest and Nacewa secured a bonus point for Leinster by the final two scores.
|Leinster Rugby Score Card|
|Aironi Rugby Score Card|