Ulster were unable to finish atop of Pool Five despite scoring five tries in their 43-6 victory over a struggling Aironi side at Stadio Luigi Zaffanella.
That was not enough to clinch top spot in Pool Four following a bonus-point victory for Biarritz over Bath. Ulster finished level on 22 points with the French club but missed out due to an inferior head-to-head record.
However, this win did secure one of two places in the last eight for best runners-up, the first time Ulster have reached the knock-out stage of the Heineken Cup since they won the trophy back in 1999.
Number eight Pedrie Wannenburg scored twice in a man-of-the-match display while tries from Andrew Trimble, Chris Henry and a penalty try wrapped up a bonus point before Simon Danielli capped the victory with a late score.
Fly-half Ian Humphreys also booted 13 points, including the opening points of the game with a third-minute penalty to help settle Ulster nerves.
But it was not until the 23rd minute that Ulster found their way over the Italian try line, having come under pressure themselves from the home team.
South Africans Wannenburg and scrum-half Ruan Pienaar combined before full-back Adam D'Arcy showed strength to hand off Aironi flanker Vickus Liebenberg and deliver the scoring pass for Trimble.
Aironi's Kiwi outside-half James Marshall got the Italians off the mark with a penalty after a break by lively wing Pablo Canavosio.
Humphreys was off target with his second penalty attempt moments later, and when Aironi full-back Julien Laharrague stepped up to knock over a long-range penalty on 36 minutes Ulster were holding onto a slender 10-6 lead at half-time.
But Ulster took control of the tie with a try inside the opening minute of the second half, Wannenburg touching down from an impressive forward drive.
Just five minutes later and Wannenburg scored again from almost the exact same move as Ulster's pack began to dominate.
Humphreys converted both and Ulster were looking more comfortable at 24-6.
Trimble threatened again, catching his own high kick and bursting through tackles forcing Aironi to concede a penalty.
However with three tries in the bag Ulster were desperate to wrap up the bonus point, and they opted to go for the line-out instead of the posts.
Their initial drive was held up short but from the resulting scrum Aironi's pack crumbled and French referee Christophe Berdos walked under the posts to award a penalty try.
Humphreys conversion broke the 30-point barrier and Ulster could relax for the final 20 minutes.
It was from another scrum that they grabbed their fifth try, pushing Aironi off their own set-piece, handing Wannenburg a perfect attacking position. The former Springbok powered forward before off-loading to replacement Henry, who picked his angle to perfection.
Wannenburg was again involved as Ulster sealed the rout with a sixth score, passing to Ian Whitten who shipped on to Danielli, handing the wing an easy finish in the left-hand corner. Humphreys missed the conversion, but Ulster were already through.
Tries: Trimble, Wannenburg 2, Henry, Danielli
Con: Humphreys 5
Aironi: 15 Julien Laharrague, 14 Pablo Canavosio, 13 Rodd Penney, 12 Gilberto Pavan, 11 Matteo Pratichetti, 10 Vickus Liebenberg, 9 Mickey Wilson, 8 Josh Sole, 7 Jaco Erasmus, 6 Gareth Krause 5 Quintin Geldenhuys, 4 Marco Bortolami, 3 Fabio Staibano, 2 Fabio Ongaro, 1 Matias Aguero.
Replacements: 16 Roberto Santamaria, 17 Salvatore Perugini, 18 Alberto de Marchi, 19 Carlo del Fava, 20 Aldo Birchall, 21 Tito Tebaldi, 22 Riccardo Bocchino.
Ulster: 15 Adam D'Arcy, 14 Andrew Trimble, 13 Nevin Spence, 12 Paddy Wallace, 11 Simon Danielli, 10 Ian Humphreys, 9 Ruan Pienaar, 8 Pedrie Wannenburg, 7 Willie Faloon, 6 Stephen Ferris, 5 Dan Tuohy, 4 Johann Muller (capt), 3 Declan Fitzpatrick, 2 Nigel Brady, 1 Tom Court.
Replacements: 16 Andi Kyriacou, 17 Bryan Young, 18 Jerry Cronin, 19 Tim Barker, 20 Chris Henry, 21 Paul Marshall, 22 Ian Whitten, 23 David McIlwaine.
Referee: Christophe Berdos (France)
Assistant referees: Jean-Luc Rebollal (France), Sébastien Minery (France)