Aironi were 20-3 adrift at half-time but showed plenty of determination and resilience in the second period, with South African flanker Gareth Krause crossing for his second try in three games.
Head coach Joe Schmidt was pleased with Leinster's application, particularly in the first half, with the four points pushing them ever closer to a home semi-final..
"It wasn't top drawer but it was a top result for us. I don't think there are any injuries coming out of the game so that keeps us pretty happy," he said, with reference to next weekend's big European clash with Toulouse.
"It was an objective (for us) to - in a sense - get into town, get the four points, and get out of town in one piece and I'm pleased that the squad managed that because I sensed at half-time that the players were starting to fade a bit.
"The heat took its toll and it was certainly a bit more humid than we have been experiencing in Dublin of late. It's important from our perspective that we don't play like that in the second half of next week's game."
McFadden landed the second of his early penalty attempts, following up on a solid set piece from his forwards. His opener in the second minute, from 41 metres out, went narrowly wide.
Leinster continued to force the issue, impressive flanker Rhys Ruddock carrying well as the visitors made the hard yards. But Aironi were level in the 13th minute following a penalty from scrum half Tito Tebaldi.
Aironi almost leaked the first try just minutes later. Half-backs Eoin Reddan and Ian Madigan got the Heineken Cup semi-finalists on the move again and Ruddock gained further ground, offloading for O'Brien who lost control of possession just short of the try-line.
Brian O'Driscoll, who captained Leinster on the day, showed his class when Leinster swept through for a 20th minute touchdown. It took a tremendous reverse pass from O'Driscoll to unlock the hosts' defence and O'Brien had a simple run-in by the posts.
McFadden added the extras for a 10-3 scoreline and Aironi, with the likes of Gabriel Pizarro and Joshua Furno getting through a lot of work, missed a chance to reply when Tebaldi struck a penalty wide of the posts.
A ruck offence allowed McFadden extend Leinster's lead with his second successful penalty and Aironi were coming under increasing pressure. Reddan threatened as he wormed his way through a midfield gap, but the Italians held out.
They were unable to do so in the 33rd minute, however, as McFadden passed for prop Cian Healy to barge past three defenders on a powerful 25-metre surge to the line. The extras were added to put 17 points between the sides.
Leinster were looking comfortable on the ball and both Madigan and young hooker Jason Harris-Wright had strong runs, testing the Aironi defence around the fringes.
Nonetheless, in and around half-time, Aironi upped their game significantly. Tebaldi watched his penalty come back off an upright just before the break and the forwards almost manufactured a try on the restart, Leinster requiring some committed tackling to keep the Italian side out from close range.
Prop Fabio Staibano was also denied a try when he dropped a pass from Tebaldi with the line in sight, following a clever break from the scrum half.
Leinster responded by building through the phases and gaining some decent territory before Aironi number 8 Nick Williams was sin-binned for a shoulder charge on O'Brien.
But Schmidt's men could not add to their tally while the New Zealander was off and with both coaches looking to their benches, the intensity dropped briefly.
With 13 minutes to go, Aironi got over for a try that their efforts merited. Krause was the scorer, crashing over from a few metres out after they kicked a penalty to touch and driven from the lineout. Marshall's conversion attempt went wide.
After that, the scoring chances dried up as Leinster completed a season's double over Rowland Phillips' outfit, keeping on track for a semi-final spot and giving themselves a winning lead-in to their Aviva Stadium showdown with Toulouse.
|Aironi Rugby Score Card|
|Leinster Rugby Score Card|