The forward who was on target against Aberdeen last weekend started from where he stopped with another stunning display over Jim Duffy’s side.
After the first-half ended goalless, the former Fulham player eventually broke the deadlock in the 62nd minute when he powered home a header from Tom Rogic’s cross.
And the French-born Malian completed his brace nine minutes later from the penalty spot after he was fouled in the area by Michael Doyle.
Odsonne Edouard put the icing on the cake at the death as the Glasgow outfit became the first club to seal a semi-final spot in the competition.
Dembele who featured from start to finish has now scored 12 goals in 30 appearances this season across all competitions.
He will be looking to continue with his impressive form in front of goal when Celtic confront rivals Rangers in their in a Scottish Premier League game on March 11.