Ifill, an £800,000 summer signing from Millwall, has been given a rough ride by the Bramall Lane boo-boys after a string of dodgy performances.
And manager Neil Warnock even dropped Ifill to shield him from the abuse he feared would shatter the 26-year-old's confidence.
But he was drafted back in for his first game in a month and shone as United comfortably disposed of George Burley's south coast outfit.
And when Warnock substituted Ifill 17 minutes from the end he earned a standing ovation from as the fans who jeered him last month were cheering him.
United blew a string of first-half chances before finally punishing Southampton's dismal defending with two goals in two minutes before the break leaving George Burley's side shell shocked at half-time.
United midfielder Phil Jagielka, voted the best player in the Championship earlier this month, celebrated the accolade with his eighth goal of the season.
The England under-21 star pounced with a left-foot shot from eight yards after Chris Armstrong wriggled down the left and squeezed across a piercing low pass.
Southampton had hardly caught their breath when they dithered again at the back and Paul Smith had to make a full-stretch save diving to his left to turn away Michael Tonge's curling shot.
But seconds before half-time the visitors paid the price for failing to deal with Rob Kozluk's monster throw in that bounced through to Ifill who drilled his shot from 10 yards past Smith.
The Yorkshire side might have stretched the lead four minutes into the second half but Tonge's snapshot from the edge of the area ran wide of the target.
And when Ifill sparkled again on 51 minutes Smith was beaten by his dipping shot from 20 yards but it smacked against the bar denying him a second goal.
On 77 minutes Neil Shipperley added the third with his 10th goal of the season as he rammed home a close-range shot after the ball pin-balled around the box and hit a post.