O'Gara kicked a nerveless drop goal to help Ireland claim the Grand Slam at the Millennium Stadium in 2009 and his match-winning heroics against Northampton deep in added time rivalled it for drama.
Munster maintained possession for some 41 phases before the fly-half dropped back in the pocket to strike his winning kick.
It undid the good work of Saints, last season's beaten finalists, who maintained their discipline in defence and scored tries in each half from Chris Ashton and James Downey.
A delighted McGahan, whose side had touchdowns from Damien Varley and Doug Howlett, said: "It was something special. The belief in the group, the way they stuck at it for 80 minutes, a lesser side would maybe not even have been in the contest.
"Ronan O'Gara's kick at the end, he's a special person to do that, to show the bottle to put his hand up and make the kick.
"This felt like a Round Six encounter during the week when it actually was an opening-round game. There is a great sense of achievement for the group at this early stage.
"This competition is like six Test matches and we are approaching it in that fashion. The win gives us a great buzz."
Northampton boss Jim Mallinder admitted his side was frustrated by the result and also by a number of decisions from referee Nigel Owens.
"The mood in the dressing room is low. The feel is one of frustration. That is the best term to use, frustration in terms of how that went," he said.
"We will look at ourselves. We did have a chance to at least score one other try, a good clear-cut opportunity when we knocked on.
"But I think the game did hinge on some crucial refereeing decisions. I felt the referee got quite a long wrong.
"I think it is as simple as that. It is difficult. It is an intimidating place to play with a cracking atmosphere.
"Sometimes you don't always get the rub of the green and I think we will look back and probably say we did not quite get it."
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