In manage at some stage in the competition, Derry hit the the front on 18 minutes whilst Junior Ogedi-Uzokwe headed in his first goal for the club.
Eoghan Stokes doubled the lead earlier than Gerardo Bruna rounded off the scoring quickly after the hour mark.
The Candystripes ended a six 12 months trophy drought via beating Cobh Ramblers to raise the trophy in September.
With a congested fixture listing ahead, Derry boss Declan Devine made numerous changes from the facet that defeated Sligo Rovers ultimate Friday.
Jamie McDonagh turned into one of those to return to the starting line-up having missed the closing four video games via suspension and it changed into the Northern Ireland Under-21 who delivered the cross for loanee Ogedi-Uzokwe to nod domestic.
First Division aspect Longford spoke back brightly to going in the back of but rarely stricken goalkeeper Nathan Gartside and shortly discovered themselves two down through Stokes’ long range power.
Stokes turned into yet again afforded an excessive amount of area within the second 1/2 and whilst his speculative effort changed into spilled via Jack Menton, Bruna become handy to turn within the rebound and register his intention for the reason that arriving at the Brandywell along Stokes in February.
The Candystripes go back to league duty towards Finn Harps on Friday having made a advantageous begin in Devine’s 2nd tenure at the helm of the facet.