Glenullin made a dream start with McNicholl in for a fourth minute goal and Eoin Bradley followed up with a point off his right.
Strokestown responded in kind with Diarmuid McGann finding the net and Kevin Finn levelling.
It remained tight for the remainder of the opening half with Cillian Bradley and Eoin Bradley pointing for the Derry side, while the Roscommon men hit back through Ciaran Regan and Shane McGinley, while a late exchange of scores left the sides locked at 1-4 apiece at the break.
Once again, it was the Ulster champions quickest out of the traps upon the resumption with Fearghal Close thumping over a score before McNicholl galloped through the middle of the Strokestown defence for his second goal.
McNicholl then turned provider, floating in a high ball which Traglach Bradley rose to fist home to cap a remarkable start to the second period.
A point from McNicholl pushed the gap out to eight and left Strokestown with a mountain to climb.
McGann grabbed the Roscommon side’s first score of the second period 11 minutes in and they were right back in it when Shane McGinley blasted to the net.
A free from McGann left just a goal between them heading into the final 15 minutes and they maintained momentum with David Neary curling over.
The wides began to mount for the Roscommon side and they would be made to pay in the 54th minute as Willie John Bradley let fly from the edge of the penalty area with a shot that arrowed into the net.
It again gave Glenullin some breathing space but Strokestown were not done with Ciaran Corcoran bringing them to within four with as many minutes left.
However, Glenullin finished with a flourish as late scores from Cillian Bradley and Dermot O’Kane preceded their fifth goal through Bradley right to seal their place in the final.