A brilliant flash of skill from centre Andy Marks gave Gonzaga victory over Castleknock at Lakeland's Park.
Gonzaga dominated the first half only to go behind to an intercept try by Castleknock out-half Daragh Henry, which he also converted in the 22nd minute. This was quite a lead on a heavy pitch.
Gonzaga's decision to take a more direct route paid off when hooker Paddy Finlay powered through the heart of a maul for a try in the 41st minute.
The winners struggled to break down a resolute defence in which Jack O'Neill was outstanding with and without the ball.
With five minutes left on the clock, Marks slipped as he received the ball, shot to his feet and immediately accelerated into space and around the last defender for out-half Conor McKeon to convert from under the posts.
Gonzaga – H Gowing; H Caslin, A Marks, K O'Connor, P Keenan; C McKeon (capt), D Lavelle; R Byrne, P Finlay, R Barry, E Byrne, C Kenny, J Carter, J Doyle, M McVey.
Castleknock – S McEntaggart (K Doyle 74); L Reilly, K Dixon, C O'Shea, L Mellett; D Henry, C Lynch; M Bennett, T Hanlon (K Murphy 56), Z Butt (S O'Hehir 41), E Murphy (A Healy 72), J O'Sullivan, D Scanlon (D Quinn 61), J O'Neill, N O'Riordan.
Ref – B Armstrong ( Leinster Branch).
Fly-half Tim Foley closed out a close contest to see Roscrea through to the final at Portarlington RFC.
This was a real ding-dong battle, Roscrea striking first through their wing Tim Carroll in the 15th minute only for Newbridge to hit back with a try by prop Hugh O'Donnell and a penalty from scrum-half Robbie Mullen to make it 8-5 at the break.
Full-back Fionn Higgins ignited Roscrea with an individual effort in the 37th minute that seemed to lift his team-mates, fly-half Tim Foley converting for a four-point lead.
They drove on from there. Leinster Schools prop Oisin Heffernan used his considerable frame to make the line to widen the gap to 17-8 soon after.
But, there was drama in store. Newbridge responded with a converted try by replacement Matt O'Connor and held the advantage of an extra man from a yellow card offence until Foley sealed the deal.
Roscrea – F Higgins; T Carroll, C Brennan, D Corcoran, R Wharton; T Foley, R Lavery; J Walsh, C Gavin (P Ennis 56), O Heffernan, K Buckley, D Trayers, M Keane, M Kelly, R Moloney (capt). Newbridge – N Delahunt; P Kyne (C Healy 56), J Howlett, J O'Neill, J Darcy; M Sutton, R Mullen; J Phelan, J Briody (C Doyle 54), H O'Donnell (E Wynne 60), T Treacy, C Nugent, R Dromgoole-Maguire, O McKnight, T Brady (capt, M O'Connor 30).
Ref – N Behan (Leinster Branch).