But the Saints needed to turn up the gears in the second-half after early tries from Paul Sackey and Michael Horak gave the Irish a 15-10 at the break.
Indeed, Northampton looked down and out after losing both locks, Ian Hunter and Jon Phillips, to first-half injuries.
Hunter was carried off with his neck in a brace after being hurt scoring Northampton's opening try while Phillips had limped away 20 minutes earlier.
Centre Chris Hyndman also went off, and his replacement Mark Tucker suffered a head wound – all before half-time.
But Saints, who gave a first Premiership start to front-row recruit and former Tonga Test captain Tevita Taumepeau, kept battling away – and it was thanks to Grayson's supreme tactical kicking that the visitor's managed to march back into the game.
In addition to his clutch of penalties and conversions, his perfectly-weighted kick into space caught the Irish defence flat-footed and allowed Bruce Reihana to cross the line.
The home side could not cope with the burgeoning pressure and failed to register a single point in the second period.
Mark Robinson added the coup de grace by sprinting over four minutes into injury-time, leaving Irish to contemplate how they could have slumped to their third consecutive Zurich Premiership home defeat.
For London Irish:Try: SackeyCons: Mapletoft 2Pen: Mapletoft
For Northampton Saints:Tries: Blowers, Reihana, RobinsonCons: Grayson 3Pens: Grayson 5