While they only trailed by three points at half-time, Ireland pulled away in a second half in which Mallett admitted his troops couldn't live with Declan Kidney's side.
Mallett has much to be proud of during his four-year tenure in charge of the Azzurri, notably a first ever defeat of France in the RBS 6 Nations.
But Italy's capacity for one-off performances was not matched by any level of consistency as they finished with the Wooden Spoon for four years running.
"History will tell whether I was a good Italian coach or not," said Mallett. "Obviously I am unhappy about the way it finished. I didn't expect as heavy a defeat.
"We got our first three points from a scrum. Had that scrum stayed together it would have helped our cause but let me say it wouldn't have helped us win the game.
"Ireland put us under pressure and a team under pressure makes mistakes.
"It was very tough for my players. I'm proud of the way they played for 40 minutes but they were thoroughly outplayed in that second half.
"The Irish were just a great deal better team than we were. Their loose forwards Sean O'Brien and Stephen Ferris were hitting the line really hard, but it wasn't just them.
"Their front five played very well. They backs controlled the game very well, never allowing us time or space.
"It's very hard to think this could have been the last match with them, it's very hard to say how this feels."
While Italy's adventure is at an end, Mallett was generous in his praise of Ireland and he has tipped Kidney's men to go all the way.
"Wales have played some great rugby and went very close to beating South Africa. It should be an exciting game," added Mallett.
"My view is that Ireland didn't come into his World Cup as big favourites because they lost four games (in the build-up).
"But they targeted Australia and ourselves as key games. Once they get into the quarters and semis, they've got a team that can knock over anyone on that side of the draw.
"No team can be confident playing against Ireland at the moment and say we've got a genuine chance of beating them. Where is their weakness?"