Two nil down at half time they staged an excellent second half fightback as they scored twice in a minute to level matters before striking for a late winner to shock the visitors.
With 15 minutes remaining goals from Aaron Connolly and Oisin Connolly within a minute of each other put Mervue right back in the game in what was a cracking cup tie.
And with just three minutes left in normal time Keelan Grainger struck for the winning goal to send the home side into round four and cap a remarkable comeback.