The early exchanges included penalty kicks for both sides as dogged defending prevented anyone from crossing the line.
However, James Wilson made the breakthrough midway through the first half to give the Saints a 13-6 lead.
Sailosi Tagicakibau narrowed the deficit for London Irish moments before the end of the first period, breaking through the Saints' rearguard for a try.
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Neither side took control in the opening stages of the second half, although Ian Humphreys brought the hosts to within two points of their opponents with a penalty.
However, Northampton were able to see the game out with six more points from Stephen Myler's boot.