The southern province side had been well in the contest in the opening quarter, Peter O'Mahony's early try and a Ronan O'Gara penalty giving them an early lead before Ian Madigan put Leinster back in front.
The boot of Jonathan Sexton moved Leinster clear but not out of sight, however it was the decisive 56th minute Brian O'Driscoll try which ultimately proved to be the difference in a game which saw the lead change hands five times.
Although Munster finished the game the stronger of the two sides Penney admits the spirited fightback was too little too late against the back-to-back European champions.
"The main downside was our error rate," said the New Zealander.
"We coughed up too much ball and missed too many tackles at crucial times. That didn't let the pressure we were building manifest itself into points.
"We were able to build pressure near the end because we were unable to hold onto the ball and build it up through a few phases. We weren't able to do that earlier in the game.
"I thought the players adapted well and got a bit of belief, which was good for us. It was a tight game, nine points, but really disappointing.
"Although there were a number of poor defensive elements in our game, the disappointing thing for me was our inability to build pressure when we had the ball.
"That is an area that we focused on this week so that is why I'm a bit disappointed with that aspect of our game."
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