St Jarlath's, the most prolific winners of the Hogan Cup, advanced to the decider with a decisive victory.
Goals from Joseph Donnellan and Conor Marsden in the opening half edged St Jarlath's one step closer to a 49th Connacht title.
Trailing by five points at the break, Cian Breheny almost single-handedly carried Summerhill back into contention with eight points from midfield, but Dara Walsh palmed the ball to the net for St Jarlath's eight minutes from time to put an end to the Sligo school's comeback hopes.
Summerhill had a chance to kick-start a late rally but substitute Michael Casey saw his added-time penalty expertly saved by James Healy to send Jarlath's into the final, where they will meet St Gerald's College, Castlebar.
The loss of centre-back Colin Brady after just 10 minutes was a devastating blow for Summerhill – who have lost the past three provincial finals.
Seconds after his departure Marsden waltzed through the centre of their defence and hit the net for the opening goal.
Donnellan's 20th-minute goal – a powerful shot to the roof of the net – gave Jarlath's a 2-4 to 0-5 lead at the break.
Breheny was in inspired form after the restart – he kicked all five of his side's points in the half – and the gap was down to four points entering the final quarter.
St Jarlath's hit back, with Paddy Madden setting up Walsh for their third goal.
Breheny landed two quick points to give Summerhill a glimmer of hope, but the lead proved insurmountable.
Scorers – St Jarlath's: J Donnellan 1-1, M Mannion 0-4 (4f), D Walsh, C Marsden 1-0 each, P Mannion 0-2; Summerhill College: C Breheny 0-8 (4f), D Kilgallon, J Gillen 0-1 each.
St Jarlath's – J Healy; O Morrin, C Reilly, J Donnellan; C Fahy, C Brady, K Murphy; D Walsh, N Daly; T Gleeson, P Mannion, J Donnellan; M Mannion, P Madden, C Marsden. Subs: C Molloy for Murphy (41), B Raftery for Daly (60).
Summerhill College – J Keegan; A Clerkin, D Cummins, L Henry; C Heraghty, D Keown, J Gillen; C Breheny, D Corcoran; S Kerins, C Stenson, S Finnegan; J Farrell, D Kilgallon, S Henry. Subs: D O'Brien for Keown (10), M Casey for Farrell (38), J Noonan for Finnegan (47).
REF – E McAndrew (Mayo).