It took a try from wing Ronan O’Mahoney in the fourth minute of injury to finally kill off a spirited defensive effort from the Pirates, who had two players sin-binned.
The Pirates had started aggressively and took a deserved lead through Aaron Penberthy’s early penalty. Munster responded just before the half-hour mark with the opening try for centre Danny Barnes following a strong break by Dave O’Callaghan, but with the conversion missed the Cornish side regained the initiative.
Two more long distance penalties from Penberthy put them 9-5 up at the interval but the second half was all Munster. With the home lineout misfiring and Penberthy forced off the field through injury Munster were able to turn the screw and their increased intensity in the second half began to tell.
The pack worked well, with flankers O’Callaghan and Niall Ronan particularly impressive and whilst the win took a long time to come it was thoroughly deserved.
Relentless pressure from the off pinned the Pirates back into their own half, with handling errors frustrating Munster in the face of athoroughly committed home defence.
JJ Hanrahan missed a penalty on the hour before a Pirates break saw Corey Hircock put in a try-saving tackle on home wing Matt Evans. Hanrahan then made a complete mess of a four-on-one overlap down the left as time ticked away.
With the Pirates cover at full stretch flanker Barry O’Mahoney thought he had finally scored in the 79th minute, only to be called back for a forward pass.
With just 60 seconds left the Pirates finally ran out of luck and O’Mahoney booked Ian Costello’s side a semi-final place as his try silenced the Mennaye Field crowd.
Scorer for Cornish Pirates: Pens: Penberthy 3.
Scorers for Munster A: Tries: Barnes, R O’Mahoney
CORNISH PIRATES: W Davies, G Pointer (T Kessell 69), T Riley, J Fatialofa (S Hill 41), M Evans, A Penberthy (T Bedford 60), G Cattle (capt); P Andrew, R Elloway (sin bin 44-54, W Tanner 80), P Joyce (Storer 60), D Barry, G Johnson (sin bin 77), K Marriott (Tanner 51-56), P Burgess, L McGlone.
MUNSTER A: S Scanlon (C Bohane 68), C Hircock, D Barnes, S Deasy (J Holland 60), R O’Mahoney, JJ Hanrahan, D Williams (G Hurley 65); J Cronin, S Henry (c) (D Casey 68), J Ryan (A Cotter 79), D Foley, I Nagle (B Hayes 76), D O’Callaghan, N Ronan (B O’Mahoney 65), CJ Stander.
Referee: I. Davies (WRU).