This turned out to be a cracking cup tie played in difficult conditions at Lisnagelvin.
Celtic drew first blood when Philip Brown lashed a shot past Smyth in the Porters goal.
Porters went all out for the equaliser and they got it courtesy of a fine strike from Gallagher.
The Waterside man again went in front, when after a superb build up Sean Paul Nash was on hand to score.
Celtic were then awarded a controversial penalty for a hand ball but keeper Darren Smyth saved the spot kick to keep his side in the match.
Porters drew level again when John Travers scored.
This equaliser was short lived as the young Celtic outfit pushed forward and Philip Brown netted his second.
Porters then threw every man forward but in the last minute young Dee Harkin scored the goal of the match when he outpaced the Porters defence and lobbed the ball over the advancing keeper to put his side into the semi-finals.
Meanwhile Derry Rovers had a comfortable 2-0 win over Brandywell Harps.
Rovers made all the early running in this tie and soon had the Harp's defence under pressure. Some fine defending kept them from scoring.
Midway through the half they went in front when a cross from Hutchman was volleyed home by Gerard McCarron.
Harps then took control for a time but never really threatened the Rover's goal.
The second saw play swing from end to end but neither keeper had a serious shot to deal with.
Six minutes from time and Billy Hutchman got the second Rover's goal when he beat the offside trap and raced clear.
In the premier division Bluebell Celtic slipped to a 4-0 hammering by Don Boscos.
Reigning champions Boscos closed to within a point of the leaders Top of the Hill after a comfortable victory over Bluebell Celtic. Ferry fired Boscos into the lead early on and this settled them. Just before the break Mickey Rogan shot home from 10 yards.
Try as they could Celtic got no change out of a strong Boscos defence. Floyd Gilmore netted a third.
With only minute remaining Paul Moore scored a fourth to wrap up the points.