The AC Milan striker netted a brace and made sure Nigel Worthington didn't taste victory in his final game in charge of the visitors
Cassano fired home a volley from Daniele De Rossi's lofted pass midway through the first half, then added a low drive soon after the break. Gareth McAuley's calamitous own goal rounded off the scoring in Nigel Worthington's final game in charge of the visitors.
The match began slowly with the only real chance falling to Sebastian Giovinco as the Italian picked up the ball on the edge of the box, but his effort was held comfortably by Maik Taylor.
The game then opened up and the visitors started to threaten the Italians, with Gianluigi Buffon forced into action to deny Andrew Little, before having to rush off his line to clear the danger after an ill-judged back pass from Andrea Barzagli.
However, Cesare Prandelli’s side should have taken the lead just before the 20-minute mark when a Giovinco corner found a stretching Giorgio Chiellini at the back post, but the Juventus man barely made contact with the ball as it flew past the goal.
It took just two more minutes for the Italians to open the scoring, thanks to a fantastic finish from Cassano. A great ball from De Rossi found the AC Milan striker inside the box, and his first-time volley left Taylor with no chance.
The Azzurri took a one-goal lead into the half-time break, and they doubled their advantage shortly after the interval when Cassano raced onto a neat ball from Alberto Aquilani before smashing in a low drive from the edge of the box.
Then, with 15 minutes to go, the home side made it three courtesy of a rather fortunate own-goal. A perfectly weighted chip from Andrea Pirlo found Federico Balzaretti, who dragged the ball across the goal forcing McAuley to steer into his own net.
This result means the Italians finish on top of Group C with an unbeaten record while Northern Ireland had already failed to qualify for next year's tournament in Poland and Ukraine after their defeat to Estonia at the weekend.