Shelbourne, somehow out of the top flight for six seasons, are back where many would argue they belong – but not without a major scare at United Park.
A bumper crowd, once they negotiated bizarrely heavy security – which featured mounted police and Garda dogs – were treated to a really entertaining game that changed utterly 12 minutes into the second half when United lost Ryan Brennan to a red card, Aiden Friel putting Shels in front shortly afterwards.
Defender Luke McNally gave Drogs hope with 13 minutes left, hope that was more or less extinguished when Oscar Brennan made it 2-1 four minutes later. Lorcan Fitzgerald bundled home a flattering third at the death.
Drogs showed immense spirit when a man and a goal down and will take huge hope going into the playoffs, while Shels will realise that they wobbled here but eventually got the job done.
For rookie boss Ian Morris, it was mission achieved at a heaving United Park, and the huge visiting support can look forward to several big Dublin derbies in the top flight in 2020.
Shelbourne had an opening on 20 minutes. Ciaran Kilduff was slipped in but Luke Farragher made it awkward for him and Luca Gratzer got down to save.
Drogheda's best move so far followed shortly afterwards with Conor Kane initially winning the ball back and Chris Lyons' shot out for a corner after a swift Drogheda move.
Lyons was booked for a late challenge but was a pestering nuisance for the visitors and headed wide from Brennan's cross; from the corner, the highly-rated McNally headed just wide, the Drogs' best chance of the first half.
Luke Byrne very bravely blocked from Sean Brennan with two minutes left and Shels did not look convincing defensively at times, Lyons remaining a big threat.
The two number 14s collided, Friel hauling down Mark Doyle on half-time, Brennan's drive just wide of Colin McCabe's post.
Former Drogs legends were honoured at half-time in a lovely gesture, the crowd warmly appreciating seeing old heroes on the Drogheda pitch again.
Tim Clancy switched to three at the back at half-time, bringing in Mark Hughes, and they began to take a stranglehold. They were well on top until everything changed on 12 minutes.
The excellent Kane crossed in and Ryan Brennan's clearance very nearly went into his own net. McCabe made a fine save, the ball hung in the air and Ryan Brennan's brother Sean was sent off for a high challenge on Fitzgerald, his second yellow of the game.
Shels predictably took control and went in front on 64 minutes, Friel driving home after a slight deflection from outside the box.
United never game up and better control from the pacy Mark Doyle could well have resulted in an equaliser, McNally having gone up front at this stage.
Initially Gratzer tipped Kilduff's header onto the bar and Drogs broke, McNally finishing like a centre-forward with a superb finish from Lyons' cross. United Park suddenly believed and Shels were reeling.
With nine minutes left, Oscar Brennan sealed matters, his nice volley on his left-foot not kept out by a despairing Gratzer dive to his left and Drogs hearts were broken by Fitzgerald in the last minute.
Drogheda United: Gratzer; Brown (Gallagher 70), McNally, Farragher, Kane; Meaney (Wixted 63), Hollywood (Hughes 46), Brennan, Hyland, Doyle, Lyons.
Shelbourne: McCabe; Friel, Prendergast, Byrne, Fitzgerald; Kabia (Fernandez 72), R Brennan, Farrell (Noone 82), O Brennan, Moore; Kilduff.
Referee: Rob Rogers.