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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.