كتب : Sherif Tarek
Emad Meteb fired Ahli ahead on the hour mark before second-half substitute Mohamed Fadl doubled the hosts' lead on 74 minutes.
Up-and-coming youngster Shehab completed the rout deep into injury time to send the home fans into ruptures.
The 19-year-old had scored a similar goal against Mansoura in the Egyptian Premier League late last month.
Ahli had to beat Ittihad by a three-goal margin in order to go through as they lost the first leg 2-0 in April.
The Cairo giants' supporters gave Ittihad a dose of their own medicine by aiming a laser beam at their players' faces throughout the game, especially during set pieces.
Possession
Ahli had the lion's share of possession throughout the game and were a bit impulsive while trying to open the scoring early in the first period.
Veteran midfielder Ahmed Hassan carved out the first scoring chance by releasing a powerful low shot from the edge of the area but missed the target.
Meteb was about to put Ahli ahead after five minutes when his shot deflected off a defender and hit the post in what turned out to be the Red Devils' best opportunity in the first half.
Ittihad replied with an organized counter attack that was competently thwarted by Wael Gomaa.
Ahli remain in control and created more scoring chances but failed to translate their threats on Ittihad into goals.
On 16 minutes Egypt skipper Hassan could have scored a sensational opener when he back-heeled a Barakat tame effort towards the net but goalie Samir Aboud kept a clean sheet for a while further.
Sayed Moawad was cautioned on the half-hour mark for what the referee considered to be a theatrical dive.
Hassan squandered a golden chance when the goal was at his mercy. He headed Sayed Moawad's cross high over from point-blank range.
Ittihad became dangerous on the counter attack minutes before the interval and had a couple of clear-cut chances, one of which was denied by the post.
Tactical Changes
Ahli boss Hossam Al-Badri replaced the misfiring Hassan with Mohamed Fadl right after the break, in an attempt to increase his team's firepower. The introduction of the former Ismaili man paid off later on.
Al-Badri, whose technical abilities were frequently questioned by many pundits this season, also sent in striker Osama Hosni.
Barakat became a winger instead of Sherif Abdul-Fadil, who made way for Hosni, and wreaked havoc down the right flank.
On 55 minutes, Meteb tried his luck but his low drive didn't disturb Aboud much.
But the Ahli ace broke the deadlock with aplomb from inside the area shortly afterwards.
Prolific striker Meteb controlled the ball with his left foot before turning around and slotting home with a right-footed drive.
Fadl came close to scoring the second upon heading a Sayed Moawad cross but the in-form Aboud parried the header away at full stretch. The in-form keeper also blocked Mohamed Abou-Treika's follow-up effort.
But Fadl repaid Al-Badri's faith when he canceled out Ittihad's aggregate lead on 74 minutes with a diving header, connecting with an inch-perfect cross from the pacey Barakat.
Substitute Hosni was close to hitting a late screamer but he wastefully overshot Fadl's pinpoint cross.
Shehab sealed Ahli's qualification in the 94th minutes, only seconds before the final whistle.