The visitors had stormed into an early lead with a try from England cap Michael Horak, the fullback going over after only two minutes.
Saracens had their Fijian fly-half Nicky Little to thank for all of their points, with the former Sharks and Pontypridd star striking four penalties, but it was not to be enough in the end, despite a last-minute score from him.
Apart from Horak's try, it was to be the boot of Little's opposite number, Everitt, that did the damage, as the Irishman struck a penalty and a drop-goal, all before his late penalty which sealed the game for the Exiles.
For Saracens:Pens: Little 4
For London Irish:Try: HorakPens: Everitt 2DG: Everitt