Mapletoft scored a brace of tries, one of which he converted, and landed a further three penalties. He has now slotted 17 of his last 19 kicks at goal.
Northampton were left to rue the amount of time it took to break down the Irish defence, and Shane Drahm's missed conversion attempt from the touchline which would have won them the game.
Their lineout was a comedy of horrors in the first half. They lost their first five throws and even Corne Krige failed to tidy things up, spilling a number of balls.
They were also guilty of basic handling errors and over-elaboration, and were reduced to risking one-man lineouts and throws to the front of the line.
When they finally got it together and surged over the Irish line at the start of the second half they were unable to ground the ball.
Even London Irish's first-half try resulted from an uncharacteristic mistake by Grayson.
The former England fly-half's kick was charged down by Irish flanker Paul Gustard, Mike Catt hacked ahead and got the luck of the bounce to send Mapletoft in.
Northampton produced a vastly improved second half which saw right wing John Rudd tackled into touch just short of the line by Irish scrum half Paul Hodgson.
They spent much of the second period in the Irish half but all they had to show for their efforts were Grayson's penalties and Human's late try.
Irish crossed the tryline again when replacement Nils Mordt set up Mapletoft for his second try in the 77th minute.
Northampton suffered a further blow when centre Chris Hyndman was carried off on a stretcher, with his neck in a brace, after being injured in a tackle on opposite number Scott Staniforth.
The delay while he received treatment, resulted in 17 minutes of injury time when Northampton's final surge was repelled by a full-stretched Irish defence.
For Northampton:Tries: HumanPens: Grayson 5
For London Irish:Tries: Mapletoft 2Con: MapletoftPens: Mapletoft 3