Maher was lively early on and made a searing break down the left wing and even though Rock's scramble defence recovered well, it was very much a warning sign.
David Hawkshaw recomposed himself after some early, understandable nerves and he controlled the tempo brilliantly with clever kicks in behind the defence.
The first glimpse of Rock's power game saw them make good ground well into Belvo territory but Kearney who was terrific, forced a crucial penalty after he made a crunching tackle.
That lifted Belvo and their supporters and shortly after they struck for the decisive blow. Rock conceded a penalty after pulling down a well-worked rolling maul and the school from the northside sensed blood.
Belvo worked the lineout well and Hawkshaw brilliantly spotted the space in behind before playing a delicate chip over the top and Maher capitalised on a defence error to score.
Hugh O'Sullivan added the extras to put his side into a 7-0 lead and they grew in confidence even more as sloppy errors continued to cost Rock.
The impressive full-back was again on target after 26 minutes when he punished Rock for their indiscipline at the breakdown as Belvo opened up a 10-point gap.
Led by their outstanding scrum-half Patrick Patterson, Rock never panicked and might have got over for a try late in the first half but for more excellent Belvo defending – Hawkshaw's strong hit on Tom Maher typifying their physicality.
The weather improved after the break and so too did Rock as the kind of passes that were going to ground in the first half, began to stick.
Peter O'Reilly eventually put them on the board three minutes after the restart as their early pressure was rewarded.
Suddenly the mistakes were beginning to come from the champions. But they regathered themselves and won a penalty deep in Rock territory and went for the corner.
The maul was worked from the set-piece and just as it looked like Belvo would roll all the way over the whitewash, the referee spotted a knock on.
Rock looked to get their potent strike runners more involved with Liam Turner carrying well in midfield but at the breakdown, they continued to come up against Kearney who was an absolute menace. The captain forced another vital penalty with 16 minutes remaining to stem the Rock pressure.
The respite was short-lived however as Rock won a penalty and sent it to the corner. Giuseppe Coyne and Cian Reilly both made the hard yards and as Belvo were camped on their own line, they somehow again forced a mistake as a knock-on won them the scrum.
The wave of attacks continued – Coyne again showing his power. Rock finally looked like they would force their way over but there was Hawkshaw who stripped the ball to win a match-winning turnover.
It was breathtaking stuff and both sides who deserved huge credit for serving up a pulsating contest in front of a large crowd in Dublin 4.
Rock threw everything they had at Belvo but they couldn't come up with the answers. Their final roll of the dice came when the won a penalty on half way which Reilly kicked it just outside the Belvo 22.
They won the lineout and again worked their way through the phases before Belvo conceded another penalty close to their line.
With the clock now ticking into the 74th minute, four minutes after the allotted time at school's level, Cian Reilly tapped and went but was stopped short.
Rock went again on the next phase and thought they were over as a loud roar went up from their supporters and with some of their players celebrating, the referee adjudged that Belvo held them up.
Cue absolute pandemonium from the Belvo players. A fitting end to a memorable final.
Belvedere - H O'Sullivan; M Donnelly, P Maher, C Walsh, J Wilkes (S Barry 44); D Hawkshaw, P Cagney; C Molloy (J Haughey 64), A McDonnell (C Byrne 61), J Robinson; G Murray, O O'Brien (D Hill 61); C Doran, M Kearney (capt), R Byron.
Blackrock - T Roche; T Maher (R Deegan h-t), L Turner, J Moriarty, D Heavey; P O'Reilly, P Patterson (R Fahy 65); G Coyne (C Delaney 64), S McLoughlin (S Moloney 51), J Byrne (T Clarkson 51); A Murphy, C Ryan (J Burke 51); J Fairley (M McGath, A Francis, C Reilly (capt).