But McGahan's side made it two wins from two Magners League games when Donncha O'Callaghan touched down and O'Gara added a conversion and penalty.
And after seeing his side secure the hard-fought victory McGahan singled out Donncha Ryan and Peter Mahony for praise.
"I was pleased with the result," McGahan said. "Last season we lost a lot of games by a similar margin.
"I thought we were tremendous in the last 20 minutes and, for me, Donnacha Ryan and Peter O'Mahony had outstanding games.
"It was, of course useful, having such a good bench. But remember these are players who are just re-integrating into the squad."
But Munster skipper Denis Leamy admitted the team needs to play better if they have any hope of making it three wins from three when they face defending champions Ospreys next weekend.
"We were really disappointed to be ahead just 6-3 at half time," he said.
"We never really threatened their line in that first period. But we showed real character when we went behind.
"When we play the Ospreys next week we have to convert pressure into points and improve aspects of our scrummaging as well as one or two points in the line-out."