Dublin City 0 Derry City 2
Dublin City's survival hopes suffered a telling blow as a tactical gamble backfired badly to leave them looking like a side now destined for relegation.
Manager Roddy Collins threw all four of his new signings into the starting 11, with Marc Kenny given the captain's armband in the centre of a five-man midfield with Danny Boxall on the right and Ronnie Henry and Scott Friel part of a three-man defence.
The new additions, not to mention the experimental 3-5-2 formation, failed to gel and Dublin had several escapes before Derry deservedly took a 17th-minute lead.
Alan Murphy should have had Derry in front six minutes earlier but shot tamely into the arms of Robbie Horgan after Seán Hargan punished a mistake by Henry to send in a dangerous cross.
But there was no reprieve from a recklessly conceded free-kick by Derek Griffin out near the left corner flag.
Gareth McGlynn sent the deadball over with wicked pace and Tony O'Connor, a double winner under Collins at Bohemians, deflected the ball into his own net at the back post under pressure from Damien Brennan.
His tactical system clearly not working, Collins took off Griffin and brought on right winger Cathal O'Connor on 34 minutes, reverting to a more conventional 4-4-2 formation.
It scarcely made a difference as Derry should have threatened again on 40 minutes.
Caretaker player-manager Peter Hutton somehow hooked the ball over his head at the end line from Murphy's deep cross to set up McGlynn, who shot over from two yards out.
A second soft goal, four minutes into the second half, sealed Derry's three points.
The hardworking Hutton stepped over a free-kick 30 yards out and Murphy's fiercely struck drive took a strong deflection off the wall to wrong-foot Horgan and enter the net.
A mistake by Clive Delaney at the other end should have let Dublin back into the match within minutes, but Gary O'Neill shot wide with only Kelly to beat.
DUBLIN CITY:Horgan; Henry, Friel, Griffin (C O'Connor, 34 mins); Boxall (Hynes, 54 mins), Tierney, Kenny, Colwell, A O'Connor; Hughes (Farrell, 51 mins), O'Neill.
DERRY CITY:Kelly; Brennan, Delaney, McLaughlin, McCallion; McGlynn (Beckett, 80 mins), Doherty, Martyn, Hutton; Murphy, Hargan.
Referee:P Tuite (Dublin).