Two second-half goals from corner-forward Shane O'Flynn inspired Salesian College to come from behind to clinch the Dublin Schools' Junior 'B' title at DCU last Friday.
The victory means the Celbridge school are celebrating an impressive double after they claimed the Senior 'B' title last month.
They overcame a determined Portmarnock side who led at the break and gave everything in pursuit of victory, but they came up short in the end.
That early lead came thanks to Shane Carthy's goal and the right-half forward was having an excellent game during the opening 30 minutes, after which the northsiders led 1-5 to 0-5.
Salesian kept in touch with Kevin Murphy sending over three frees and Jordan Morrissey operating well at full-forward.
But O'Flynn led the way after the break, notching up 2-5 overall in an impressive display.
The corner forward's first goal 15 minutes from time edged Salesian in front, but Portmarnock took control of the ball and had a spell of pressure.
They couldn't make it pay with Salesian defenders Jason Doran, Patrick Brophy and substitute Rory Feehan keeping their opponents at bay.
Midfielders Dylan Corcoran and Conor Brady reasserted their control as they pushed to close out the win.
And they struck again through O'Flynn who took his second goal well and gave his side an insurmountable lead.
Portmarnock can hold their heads up after the final defeat, with Jack Watson, Niall Murphy, Daire Foley, Rory Deasy and Niall Conlan all contributing to their performance.