First-half goals from Gary Twigg and Darren Murray gave the Ports victory over a Donegal Celtic side facing a Premiership relegation play-off.
Twigg netted for the visitors on six minutes with a close-range header from Sean Mackle's cross.
Ross Redman was the provider for Murray's headed finish before DC pulled one back after 36 minutes when Kevin Hughes nodded in.
Ports striker Kevin Braniff was denied by the woodwork in the second half.
Mackle set up the opener and only a last-gasp header from Ryan Deans prevented the midfielder from netting minutes later.
Murray made it 2-0 after 34 minutes but the hosts made a quick response when Hughes got down well to head home.
Hughes missed the chance to equalise early in the second half by heading over before Ports forward Braniff struck a post.