Conor Sammon's brace helped Derry to beat their north-west neighbours and claim a place in the Eircom League of Ireland Cup quarter-finals.
Former Harps player Kevin McHugh twice went close for City before his cross was prodded home from six yards by striker Sammon on 32 minutes.
Niall McGinn cut inside before firing past keeper Gavin Cullen to make it 2-0 five minutes after the break.
Sammon netted from close range after 59 minutes to seal the victory.
McHugh and Owen Morrison were involved in the build-up for the third goal.
Derry had gone three games without scoring before the Brandywell derby encounter on Monday night.
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