Sure-footed out-half Bill Duggan kicked Cistercian Roscrea through to a quarter-final date with either Terenure or Gonzaga.
And, on this evidence, they will be no pushover. They had a splendid game plan, which put a second-half stranglehold on Castleknock, keeping vaunted Ireland U-18 Schools open-side flanker Jack O'Neill under relative control.
Out-half Duggan was the architect of it all behind a compact, cohesive pack, which was led superbly by No 8 Colin Moloney and fleet-footed hooker Denis O'Dwyer.
Duggan kicked three penalties and also added the conversion to Leinster U-19 centre Conor Finn's late try.
Roscrea -- P O'Dwyer; C Brennan, S Layden, C Finn (capt) (M Fitzgerald 69), M Mellotte; B Duggan, G Gallagher; G Fallon, D O'Dwyer, C Farrell (O Heffernan 61); A McGrath, R Maloney; A Henderson (R Mooney 63), E Quirke, C Moloney.
Castleknock -- M Mellett; S Carroll (A Boswell 48), T Shields (J McCormack h-t), T Farrell, E Quinn; D Henry, R Gleeson; M Briody, D Barnes (D Perry 48), O McKevitt (M Bennett 48); C O'Riordan (capt), P Nalty; S Condon (A Mu'azzam 48), J O'Neill, J O'Brien.
REF -- S Gallagher.