Despite the final scoreline, both side’s had chances before fly-half Riki Flutey bagged a penalty from 30 yards.
The penalty came following referee Wayne Barnes having to separate the two teams when a fight broke out after Northampton flanker Paul Tupai high-tackled home scrum-half Paul Hodgson.
And, just before the half-hour, wing Delon Armitage just got a hand down on a kick-and-chase from 40 metres to grab the try that, along with Flutey’s conversion, put the Exiles into double figures.
Throughout the game, New Zealand fly-half star Carlos Spencer tried to break the Irish line and set his backs going and it was the All Black who put Northampton on the board when he boomed over a 40-metre penalty.
Northampton were mounting a couple of attacks down the right wing as John Rudd and full-back Robbie Kydd looked to outwit Irish winger Sailosi Tagicakibau.
One of those enterprising efforts, though, unexpectedly led to the Saints conceding a second try.
Rudd stormed into the Irish 22 and passed to scrum-half Mark Robinson only for the half-back to drop the ball. Centre Nils Mordt, though, grabbed the loose ball and began a counter-attack down the right.
He linked with Armitage who went galloping over the half-way, cutting back towards the centre of the pitch before sending a pass to Nick Kennedy, who slipped the ball to David Paice for the hooker to run as fast as his legs would
carry him from 35 yards to just get the touchdown.
The Saints responded and came close through Tupai and Sam Harding, the persistent pressure leading to England Under-21 international wing Topsy Ojo getting a yellow card when referee Barnes just about had enough of the penalties
the home team were giving away.
Despite having the numerical advantage for 10 minutes, Northampton could not find a way through the Irish defence.
When the hosts got inside the opposition 22 again, number eight Juan Leguizamon drove into a tackle and the ball came back on the Irish side. Catt cleverly went left where Paice was waiting to receive. The hooker then pushed the ball out to Tagicakibau for the winger to pounce for the try.
Catt also saw the ideal opportunity to push the boat out when he attempted a long-range drop goal and saw it go arrow straight through the posts.
There was only pride left for Northampton but a bonus point for scoring four tries for the Exiles. They thrust a few times in order to get the extra point but were frustrated until the last two minutes of the match.
That saw Kennedy win a clean line-out ball on the Northampton 10-yard line. It was swept to the middle for Catt to charge off right. Catt got to 15 yards out before giving it to replacement Gonzalo Tiesi who put Ojo in for the vital fourth touchdown.