The Irish striker notched twice in the first 14 minutes and, after Magaye Gueye’s effort at the end of the first half had halved the deficit, substitute Jonathan Parr wrapped up the win late on for the Tractor Boys – who moved up to second in the table.
Millwall began brightly as new signing Diego Fabbrini, on loan from Watford, dragged the ball wide of the left post from inside the box after his initial shot was blocked.
But less than 90 seconds later Stephen Hunt lobbed the ball over the defence allowing Noel Hunt, who signed a permanent deal with the club this month, to latch onto the loose ball and finish under David Forde from close range.
Hunt tapped in for his second of the game eight minutes later after a dangerous cross found its way to Kevin Bru.
Bru saw his shot parried straight into the path of the Ipswich forward, who gladly helped himself to his third Championship goal of the season.
Millwall’s new-look defence – containing debutant Shaun Cummings and 19-year-old academy graduate, and captain for the day, Sid Nelson – looked shaky as the visitors peppered the box with testing crosses.
The hosts did eventually play their way into the game but Jimmy Abdou screwed his shot horribly wide after Gueye’s lay-off on the edge of the area.
And the struggling Lions grabbed a lifeline on 42 minutes when Stefan Maierhofer flicked on for Gueye, who saw his shot creep in off the post.
Millwall began the second half with the crowd behind them as they looked to earn their first win at The Den since a 1-0 triumph against Cardiff on October 25.
And they continued to threaten. On 57 minutes Gueye put in a dangerous corner towards centre-back Byron Webster, who could only direct his header narrowly wide.
The Ipswich squad had been hit by illness and it looked like the home team would take advantage.
Gueye was proving the main threat for Ian Holloway’s men and the French winger saw his 20-yard free-kick go narrowly over Bartosz Bialkowski’s crossbar.
Ipswich tried to counter Millwall’s second-half superiority and Tractor Boys manager Mick McCarthy sent on new signing Freddie Sears.
The striker almost made an immediate impact as he broke away from the Millwall defence, but he saw his shot bravely blocked by Nelson.
Sears did make a decisive contribution on 75 minutes, though, as his pass set fellow substitute Parr free to comfortably slot past a helpless Forde – extending Ipswich’s unbeaten away record to seven games in all competitions.