Jim McLaughlin's side secured the remarkable achievement, which still to this day has yet to be equalled, thanks to Felix Healy's winner against Cork City, in the FAI Cup Final, on May 7, 1989.
McLaughlin's men, who had already won the League title and League Cup, became the first Derry side to win the FAI Cup in the club's history with the win over the Rebel Army, at a packed Dalymount Park.
That Derry squad, which only featured 17 players during their league campaign, produced a host of top league of Ireland greats from that side and cemented their place in Brandywell folklore.
While the Candy Stripes have on a number of occasions missed out on completing another treble, Vinny Perth's Dundalk were a penalty shoot-out away from joining City, but they fell at the final hurdle in last year's FAI Cup Final, when Shamrock Rovers defeated them at the Aviva Stadium, last November.
1. Derry City Treble
Derry City players Noel Larkin (left) and Paul Carlyle during the lap of honour after their 1989 FAI Cup Final win over Cork City, at Dalymount Park.
2. Jim McLaughlin
Derry City manager Jim McLaughlin celebrates guiding his home town club to treble glory in 1989.
3. Derry City
Derry City celebrate securing the FAI Cup and completing the treble in 1989.
4. Derry City
Derry City supporters in joyous mood ahead of the FAI Cup Final against Cork City, which took place at Dalymount Park, in 1989.