The young out-half delivered a mature performance to outshine his opposite number Ronan O'Gara, who missed three of his four penalty attempts - including two in the opening six minutes.
It was an off-colour display from the usually reliable Corkman and his team-mates, despite some encouraging play from JJ Hanrahan and stand-in captain Mike Sherry, had a night to forget as a collective.
Unforced errors and poor decision making blighted Munster's play, although they did create a late opening for winger Denis Hurley to touch down to the left of the posts. But time was already up and there was no chance to push for a losing bonus point.
Both sides had Six Nations squad members in their line-ups. Munster were boosted by the inclusion of Donncha O'Callaghan and O'Gara, while the Scarlets included Josh Turnbull and Liam Williams.
O'Gara had a testing early kick after the Scarlets were penalised on halfway, but he left his effort hanging short of the target.
It was not a great start for the out-half and things did not improve. The Scarlets were caught offside in front of their posts, but again O'Gara failed with his kick from 35 metres out, the ball rebounding off the left upright this time around.
While O'Gara struggled to find his range, Owen Williams opened the scoring at the other end with a sweetly-struck penalty after 13 minutes.
Williams was short with another attempt inside his half, but he split the posts with his third shot to double the Scarlets' tally by the end of the first quarter.
Eventually Munster got a foothold in Scarlets territory, with O'Gara finally on target with a 29th minute strike from 25 metres out after the home side were penalised at a scrum.
But that was cancelled out by the in-form Williams, who kicked a fine 55-metre penalty to give the Scarlets a 9-3 advantage at the interval.
Munster's discipline let them down at the start of the second half with winger Johne Murphy conceding a penalty for a clash with Phil John, but fortunately for the province Williams missed the resulting kick.
But Munster still continued to struggle. Williams kicked his fourth penalty, while O'Gara missed his third from four attempts.
It got even worse for Penney's charges as hooker Sherry was sin-binned in the 56th minute, allowing youngster Williams to kick a fifth penalty as the Scarlets led 15-3.
And down to 14 men Munster went further behind to an Williams drop goal just past the hour mark, with Simon Easterby's men seemingly quicker to the breakdown and hungrier for work.
They were good value for their win, following up on their 13-6 success in Cork back in November to complete their first season's double over Munster since the 2003/04 season.
Casey Laulala, Hurley and Murphy were ever willing runners, but the visitors were unable to really stretch what was a committed and well organised Scarlets defence.
Munster, at least, had the final say on the scoreboard after man-of-the-match Williams blotted his copybook by getting sin-binned.
Hurley went over for the only try of the contest, rewarding a bloodied O'Gara, replacement scrum half Cathal Sheridan and Ian Keatley for some patient and precise build-up play.
Unfortunately for Munster this glimpse of what they could do in the opposition's 22 came far too late. O'Gara's successful conversion was the final kick of an entertaining if error-ridden encounter.
|Scarlets Score Card|
|Munster Rugby Score Card|