Scarlets overcame the driving rain to claim their first win at Musgrave Park in eight visits and a first victory anywhere over Munster since the 2007 European quarter-final.
Drawing 3-3 at the interval, Munster had already lost the services of Ronan, whose comeback after more than 10 months out with cruciate knee ligament damage and then a groin injury lasted just 35 minutes in Cork.
A clash of heads with Scarlets lock Jake Ball left Ronan needing a stretcher and neck brace following more than five minutes of treatment on the pitch.
“It looked like a relatively serious neck injury but we’ve got information coming in that it’s not significant and we’ll just have to wait and see and hope for positive news over the next wee while,” Penney said.
The Munster head coach had plenty else to rue from the first half as the home side struggled to tune in to French referee Laurent Cardona’s officiating at scrum-time, wasted valuable try-scoring opportunities and had another ruled out when Ronan’s replacement in the back, debutant CJ Stander, saw his effort judged short of the line by the TMO.
“In those conditions when the opportunities arise you’ve really got to be able to take them and we probably had two clear try-scoring opportunities in the first half that we didn’t nail.
“We got a bit frustrated at scrum time and we lost our way a little bit there and that really cost us too today.”
The Welsh region had been happy to go in at the break with the scores level at a penalty apiece and edged in front with a 68th-minute Aled Thomas penalty before capitalising on poor Munster handling of the slippery ball to snatch a 70th minute try through wing Andy Fenby, converted by Thomas.
Luke O’Dea saw a pass in midfield slip out of his hands and when Fenby kicked towards the Munster line, the otherwise impressive Felix Jones failed to gather and the ball wriggled out of his grasp and over the white paint where the Scarlets wing applied downward pressure for a crucial five points.
With Penney’s side pressing to get back on terms, the Munster pack continued to struggle at scrum time as Cardona whistled repeatedly, not least for a series of early engagements.
“I think a large degree of it was our own problem,” Penney said.
“It’s an area of the game that we’re endeavouring to get better and we’ve made some improvements but I think we might have taken a slight step back today in some areas.
“Also, you’ve got to be smart enough to be able to read what the referee’s interpretations are going to be. He was relatively consistent so we’ve got some work to do there.”
Of the positives, Penney praised former Springbok U20 back rower Stander, who had left behind a South African summer for the delights of a debut in cold and wet Cork.
“Yeah, he did well. It was hard at times to identify exactly who was doing what and to who but he contributed a great deal to the effort and I thought there was some good endeavour going into those terrible conditions in the second half.”
In ever worsening conditions Hanrahan showed his value off the bench as a replacement for Keatley. With time up and the chance gone of avoiding a first defeat at Musgrave since Glasgow won there in 2008, Hanrahan slotted the penalty that closed the deficit to seven points and earned his side a bonus point.
“It was great to be able to get that after the disappointment of realising we weren’t going to take four [points] to sneak up. To take a bonus point at the end was a great credit to young JJ to be able to put the ball between the uprights in really atrocious conditions.”
MUNSTER: F Jones; D Howlett capt, C Laulala, J Downey, L O’Dea (D Hurley, 70); I Keatley (JJ Hanrahan, 70), D Williams (P Stringer, 73); W du Preez (M Horan, 68), M Sherry (D Varley, 68), S Archer (BJ Botha, 45); I Nagle (D Foley, 70), B Holland; Dave O’Callaghan, N Ronan (CJ Stander, 36), P Butler.
SCARLETS: M Stoddart (D Newton, 58); N Reynolds, G Maule, G Owen (A Warren, 75), A Fenby; A Thomas, G Davies (R Williams, 75); P John (S Hopkins, 75), E Phillips (K Myhill, 75), J Adriaanse (S Lee, 54); J Ball, R Kelly; G Earle (J Snyman, 63), J Edwards – captain (D Thomas, 45), C Price.
Referee: Laurent Cardona (France).