Leinster winger Sam Coghlan-Murray crossed 10 minutes from time to put Leinster 'A' into the B&I Cup final, but also gave Girvan Dempsey's men their first win in four against Munster 'A' at Dooradoyle.
The defending champions had the chance to steal victory in the final minute, but fly-half JJ Hanrahan missed his first kick of the day after Marcus Horan had forced the penalty from a Leinster defensive scrum in possibly his last act in a red shirt.
Initially it looked as if Noel Reid's and Hanrahan's battle from the kicking tee would decide the derby, but Coghlan-Murray finished off a flowing move across the line to give Leinster 'A' the victory.
Leinster had led by six points at the break, only for Hanrahan to level affairs by the 64th minute with two more penalties, but the UCD man's try proved decisive, especially when Hanrahan slotted his fifth kick five minutes from time to put just two points between the sides.
After drawing level with two early penalties and falling behind to Coghlan-Murray's try, the home side looked likely to force an unlikely win.
But with Hanrahan's kick missing the target, it is Leinster 'A' that will met the winners of today's semi-final between Newcastle Falcons and Bedford.
Munster 'A' – S Scanlon; D Barnes, I Dineen, C Bohane, R O'Mahony; JJ Hanrahan, D Williams; J Cronin, S Henry, A Cotter; D Foley, I Nagle; N Ronan, B O'Mahony, CJ Stander. Reps: M Horan for Cotter (52), S Dougall for B O'Mahony (52), D Casey for Henry (70), S Deasy for Bohane (70).
Leinster 'A' – S Crosbie; A Boyle, A Byrne, N Reid, S Coughlan-Murray; C Marsh, L McGrath; J O'Connell, A Dundon, M Bent; T Denton, M Flanagan; B Marshall, C Gilsenan, L Auva'a. Reps: D Leavy for Marshall (23), T Byrne for Denton (74), M McGrath for Crosbie (76), A Griggs for Boyle (79).
Ref – I Tempest (RFU).