But second half touchdowns from Niall Morris and David Kearney, allied to 11 points from man-of-the-match Isa Nacewa, saw a youthful Leinster grind out a home victory.
Bottom side Aironi still made sure they took something from the match, with Paolo Buso's last-minute penalty securing them their first league point earned outside of Italy.
Kevin McLaughlin, who started in Leinster's second row, failed to gather the kick-off and Kaine Robertson drew a second minute penalty which Tito Tebaldi slotted from five metres in from the right touchline.
The opening half was less than memorable, with handling errors, crooked lineouts and overcooked kicks blighting general play.
Nacewa popped his second penalty attempt through the posts, after an infringement from Fabio Staibano, and Leinster got straight back on the attack.
There was some incisive running from Brendan Macken and Kearney, but a forward pass from Stephen Keogh to Eoin O'Malley curtailed a promising move.
Teenage centre Macken's debut was cruelly ended by a broken collarbone, disrupting Leinster's plans further as they were already without 11 Ireland internationals.
Tebaldi missed two penalties - the first smacking the right hand post - and Ian Madigan began the second quarter with a long range miss.
Aironi's aggressive defence kept Leinster out of scoring range and the Italians enjoyed their best spell before the break. Paul O'Donohoe's loose pass set Aironi off on a breakaway which ended with Horacio San Martin being caught just short of the try-line.
The visitors used turnover ball again to pile forward through their pack. Williams, the former Munster back rower, got over the line, but television match official Peter Ferguson said the grounding was inconclusive.
However, a minute later, Williams made certain of the try as he broke off a five-metre scrum, flattened Madigan and crashed over to the right of the posts. Tebaldi added the conversion for a seven-point lead.
Leinster's blushes were spared in the final minutes. Replacement Morris prevented a second try for Aironi, bringing down Jacob Erasmus as the flanker tried to finish an interception in the right corner.
Leinster upped the intensity for the start of the second half and Nacewa's close-in penalty cut the gap to 10-6.
Then, in the 46th minute, there was a powerful burst on the left wing by Richardt Strauss whose one-handed offload invited Morris through and he outpaced Danwel Demas, from outside the 22, for a neat finish under the posts.
Nacewa's simple conversion was cancelled out by a terrific drop goal from deep from the influential Buso, and it was game on again at 13-13.
Nacewa kicked his third successful penalty to make it 16-13 to the hosts and after full-back Buso curled a penalty effort wide of the uprights, Leinster roared forward.
The 20-year-old Rhys Ruddock, the province's youngest ever captain, led the charge in a side containing eight players aged 23 or under.
One of the youngsters, winger Kearney, gathered Shane Jennings' deft pass out of the tackle and brushed off James Marshall's challenge for the decisive score in the left corner.
Back came Aironi to deservedly grab a losing bonus point through the boot of Buso, who took over the place-kicking duties following Tebaldi's departure.
Speaking afterwards, Isa Nacewa said: "There were lots of young guys out there and we came out in the second half and showed a good bit of character, but we made hard work of it at times.
"We were lucky to get away with it at the end, but we're happy with the win. We'll take the fight-back as a positive but we have a lot of hard work to do for a big game against Cardiff next week."
|Leinster Rugby Score Card|
|Aironi Rugby Score Card|