Derry missed the chance to go top as new leaders Cork won this League of Ireland Premier Division clash.
Cork keeper Dan Connor made a superb save to keep out Mark Farren's header before Dan Murray headed in the only goal from close range on 39 minutes.
Derry's Mark McChrystal was sent-off in the 52nd minute for two yellow cards while Cork's Stephen O'Donnell was dismissed for dissent on 90 minutes.
James McLean netted for Derry in injury-time but it was ruled offside.
The visitors made a good start with Connor saving from Sammy Morrow's shot.
Cork hit back and Fahrudin Kuduzovic went close twice in four minutes.
Murray finished after Greg O'Halloran headed the ball across the goal from a free-kick.
Despite being a player short Derry pressed for an equaliser and Eddie McCallion fired just wide from 25 yards.
Stephen Kenny's side remain in third place but they now trail Cork by five points.
Elsewhere, Galway beat Bray Wanderers 3-0, Drogheda United and Dundalk drew 1-1 while St Pat's Athletic and Sligo Rovers played out a 2-2 draw.
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