Northampton responded with two Stephen Myler penalties to close the gap to two points, 8-6 to Munster.
O'Gara found Doug Howlett out wide to put him over in the right corner after 23 minutes, O'Gara once again missing the kick, 13-6.
Northampton responded with another Stephen Myler penalty and then in the 39th minute centre Jon Clarke found space to score the visitors first try, Myler adding the points putting Northtampton ahead 16-13 at half-time.
On the return it was Jean de Villiers who put Munster back in front, with ROG converting giving Munster a four point margin.
After another penalty from Myler and two from O'Gara, Doug Howlett found himself going over the line for his second try of the evening.
With O'Gara converting, Munster sealed the deal securing their ninth Heineken Cup semi final appearance, defeating Northampton 33-19 in Thomond Park.
Tony McGahan after the win "We were really composed out there and just held it together. It was a wonderful day to play some open rugby and the opportunities were there for us to take."
"Our forwards were tremendous today and with the loss of someone like Paul we really had to step up to the mark and each and every one of them did that out there."
"I thought O'Gara was at his best today with his game management and overall control of the game."
James Coughlan after his first Heineken Cup start commented "I thought we started really well, there was intensity there and everyone wanted to step up to make a hit."
When asked if O'Connell gave him any advice - "Paul had a word with pretty much everyone. It was a massive opportunity for me and I didn't want to let anyone down and I think everyone felt that."
"It was hugely intense out there, every little thing is a contest, all the break-downs, all the charge-downs, you have to fight for everything."
Coughlan on his starting call-up, "When I got the call from Tony I thought I'd been dropped completely! But it turned out to be the other way around so that was good!", and after receiving his first Heineken Cup start Coughlan's now hoping for a battle with Harinordoquy come the May weekend!
Munster: Paul Warwick; Doug Howlett, Keith Earls, Jean de Villiers, Ian Dowling; Ronan O'Gara, Tomas O'Leary; Marcus Horan, Jerry Flannery, John Hayes; Donncha O'Callaghan, Mick O'Driscoll, Alan Quinlan, David Wallace, James Coughlan. Replacements: Damien Varley, Julien Brugnaut, Tony Buckley, Billy Holland, Nick Williams, Niall Ronan, Peter Stringer, Lifeimi Mafi.
Northampton Saints: Ben Foden; Chris Ashton, Jon Clarke, James Downey, Bruce Reihana; Stephen Myler, Lee Dickson; Soane Tonga'uiha, Dylan Hartley capt, Euan Murray; Courtney Lawes, Juandre Kruger; Phil Dowson, Neil Best, Roger Wilson. Replacements: Brett Sharman, Regardt Dreyer, Brian Mujati, Ignacio Fernandez Lobbe, Mark Easter, Alan Dickens, Shane Geraghty, Joe Ansbro.