Dan Biggar, Hanno Dirksen, Andrew Bishop, Rhys Webb and Kahn Fotuali'i all scored tries, with Biggar adding four penalties and four conversions.
In response, Munster's Ian Keatley grabbed all of his side's points with a try, conversion and a penalty.
But the Irish side were rarely in the game, and Tandy said he felt his team's defensive work had set-up the comfortable win.
"I thought it was a great performance and to beat Munster is always good," he said.
"I thought the defence was key, as Munster did put us under pressure but we turned the ball over and when we did that we did execute and secure the points.
"Even when we were two or three tries up still the defence didn't want to give Munster any easy points and made them work for everything.
"It was a really good night for us.
"I thought the supporters were unbelievable, but we can't get ahead of ourselves as we have a final now to hopefully go and win."
RaboDirect man of the match Paul James, who is leaving Ospreys to join Bath in the summer, added: "When you play teams like Munster, with the skills they have got, you have got to have your defence on top.
"I think we clicked and the boys showed just what we can do."
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