Quins will make the short trip home wondering why they failed to turn on the gas until the final quarter of the match.
It was the Exiles who looked the more inventive in the early stages, and attempted to put some width on the ball at every opportunity.
It could have been due to the sweltering conditions, or the daunting crowd of 50,000 fans at HQ, but neither side looked very comfortable on the ball, and the stop-start first half ended 6-3 in favour of the Irish.
Geoff Appleford's 54th-minute try put Irish on course for victory, with an opportunist touchdown from England's Sackey near the final whistle sealing the result coach Gary Gold's men deserved.
Quins staged a revival late in the second half, but by then there was too much ground to make up.
In the end, with the result already beyond doubt, Quins opted to go for posts in an attempt to salvage a least a bonus point for finishing within seven points of their victors – and new signing Jeremy Staunton duly delivered the scant rewards with the ensuing kick.
For London Irish:Tries: Appleford, SackeyCon: CattPens: Everitt 2
For NEC Harlequins:Pens: Staunton 3, Dunne
Yellow card: Everitt (London Irish, 41)