Northampton went into a six point lead with 5th and 12th minute penalties before Ronan O'Gara opened Munster's with a penalty from the ten metre line shortly after.
Then when Munster's scrum came under severe pressure Romain Poite awarded a penalty try that Lamb converted.
O'Gara reduced the deficit at the start of the second quarter with another penalty after a sustained bout of Munster pressure and in the 28th minute the home side had to be happy to concede just three further points after another sustained bout of pressure.
From the restart Lamb shot over a neat drop goal but the end to end stuff continued with first Simon Zebo and then Peter O'Mahony stopped short of the line before BJ Botha crashed over with O'Gara's conversion bringing the scores sixteen points apiece.
Munster raced into the lead early in the second half with tries from Johne Murphy and Simon Zebo the first converted by O'Gara but then the Saints were awarded their second penalty try and Conor Murray and Lee Dickson were both yellow carded.
O'Gara and Lamb swapped penalties before O'Gara extended Munster's lead with a 68th minute penalty and Simon Zebo's intercept try from inside his own half guaranteed Munster a home quarter final.