Master McGrath guides Pat's past youthful Hoops
Shamrock Rovers 1 St Patrick's Athletic 3
A sublime goal and an assist from Jamie McGrath saw holders St Patrick's Athletic come from behind at Tallaght Stadium to book their place in the final.
Liam Buckley's side meet Limerick in the final on September 17 at a venue to be decided.
Rovers, with 18-year-olds Trevor Clarke, Shane Hanney and Sean Boyd impressing, were rewarded for their dominant start with the lead goal on 34 minutes.
The industrious Boyd won possession from a block tackle with Rory Feely near halfway to put Dean Clarke in the clear.
And the striker ran on to turn Darren Dennehy before drilling a low shot through the hands of Conor O'Malley to the net for no more than Rovers deserved.
Scarcely in the game, Pat's finally threatened for the first time on 43 minutes when Christy Fagan's header was deflected for a corner after good interplay on the left from McGrath and Mark Timlin.
Rovers were then left kicking themselves as a third successive corner resulted in Pat's levelling two minutes into added time.
The marking was negligent as Dennehy used his physique to meet Timlin's delivery and head home off the back post.
Despite again enjoying much of the play from the resumption, poor defending cost Rovers a second goal on 57 minutes.
Lee Desmond was allowed to surge through from midfield before deftly slipping the ball to McGrath to score with a delightful first-time rising shot to the top corner past Barry Murphy.
It should have been 3-1 six minutes later. Again the defending was questionable as Fagan got to the near post to meet Timlin's cross only to power his header over the top.
Chasing the game, Rovers' Gary McCabe flashed an angled shot across goal before Boyd was off target with a header from a corner.
But Saints' McGrath would have the final say, threading a superb pass through for the run of Fagan, who rounded Murphy before rolling the ball to the net for the clinching third goal on 79 minutes.
Shamrock Rovers - Murphy; Hanney (Madden 64), Webster, O'Connor, T. Clarke; Cregg, McCabe; G Brennan (Doona 76), Miele; Boyd, D. Clarke (Shaw 64).
St Patrick's Athletic - O'Malley; Barker, Feely, D. Dennehy, Bermingham; Verdon (Kelly 11), Barker; Kinsella (Byrne 78), Timlin (B Dennehy 89), McGrath; Fagan.
Ref - DMcKeon (Dublin).