Munster left it late to book their place in the last four as they needed a try in the fourth minute of injury-time from wing Ronan O'Mahoney to finally overcome a stubborn Cornish Pirates side in Penzance.
The Pirates looked set to hold on against the cup holders as they withstood wave after wave of Munster attacks in a thrilling finale, but once Gary Johnson had been sin-binned for killing the ball close to his own line, their tiring pack finally ran out of numbers in defence.
It was a deserved win for Ian Costello's side. After falling behind to an early Aaron Penberthy penalty, Munster took the lead on 25 minutes as flanker Dave O'Callaghan scythed through the defence and offloaded to Danny Barnes for the opening try.
The Pirates went back ahead with another long-range effort from Penberthy, and with JJ Hanrahan again off target from a penalty, they increased their lead to 9-5 just before half-time as Penberthy struck again.
Incessant pressure after the break from Munster kept the Pirates pinned in their own half.
Flanker Barry O'Mahoney had a try cancelled out in the 79th minute for a forward pass but deep into stoppage-time Ronan O'Mahoney finally landed the killer blow.
Cornish Pirates – W Davies; G Pointer (T Kessell 69), T Riley, J Fatialofa (S Hill h-t), M Evans; A Penberthy (T Bedford 60), G Cattle (capt); P Andrew, R Elloway (W Tanner 80), P Joyce (R Storer 60); D Barry, G Johnson (77); K Marriott, 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 (capt, 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.
Ref – I Davies (WRU)