Replacement Danny Barnes scored Munster's only try, with fly-half Ian Keatley adding the rest of the points but McGahan's men have now fallen behind Ospreys in the table.
And McGahan was the first to admit that his side got exactly what they deserved.
"We were comprehensively beaten," said McGahan. "It was a harsh lesson because they really did have a great attitude and we were second best when it mattered.
"They played in all the right areas and they ran with venom and they deserved the win."
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