Trillick edged out Glenties by a single point at Ballybofey, 2-9 to 0-11, to book a semi-final against Scotstown.
Niall Gormley scored a third-minute goal, Glenties had Anthony Thompson sent-off on 12 minutes for a headbutt, and Dermot Molloy went off injured.
Daire Gallagher fired in the second goal as the Tyrone champions secured a 2-4 to 0-4 lead at half-time.
Ciaran Thompson rifled into the corner of the net from 20 yards but despite dominating Glenties fell just short.
All the odds seemed stacked in Trillick's favour in the early stages as Gormley reacted quickest to fire past Stephen McGrath into the net after a free had cannoned off the upright.
Thompson's dismissal and Molloy's injury proved further setbacks for the Donegal men, who trailed at the interval despite bossing most of the possession.
Thompson's fine goal brought his side back into the game but the impressive Brennan played a significant role in keeping the victors in touch.
Late points from Richard Donnelly and Brennan proved enough as Daragh Gallagher squandered a chance to equalise in the dying moments when his free went wide.