Superbly executed goals from Eamon Zayed and Shaun Williams saw Sporting Fingal's quality come to the fore as Shelbourne face a third season in the First Division.
FAI Cup finalists Sporting now host Bray Wanderers at Morton Stadium on Friday night before the return leg of the play-off final at Carlisle Grounds on Monday.
Shelbourne were the hungrier, more up for it side from the start, taking the game to Sporting with David McAllister heading Michael Synnott’s cross straight at keeper Darren Quigley after just three minutes.
Sporting had both Brian Gannon and their captain Stephen Paisley booked early on as they struggled to contain Shelbourne’s direct approach.
Richie Baker was narrowly off target from a free-kick on 17 minutes before Sporting gradually came into it, twice almost punishing Shelbourne with their pace on the counterattack.
Conan Byrne had his fierce shot bravely blocked by Shelbourne captain Damien Brennan before Zayed broke following a mistake by Synnott.
Byrne got possession on the right to cross for Lorcan Fitzgerald who cut in to shoot wide off the outside of Dean Delaney’s right-hand post.
Having found their range, Sporting blew Shels away with two superb goals shortly before half-time; the opener, on 39 minutes, an absolute gem.
Byrne showed good feet before putting Gary O’Neill away on the right. O’Neill turned Alan Keely inside out to cut in and cross for Zayed to volley sublimely to the net.
It got worse for Shelbourne within two minutes when Williams punished further poor defending to advance unchallenged and drill a left-foot strike into Delaney's bottom left corner from 20 yards.
It should have been 3-0 ten minutes into the second half when Shelbourne were all too easily carved open again.
Shaun Maher waltzed forward to link with Zayed who put O’Neill in. Only the finish was lacking as the shot was wide across goal.
Shelbourne had to respond and they laid siege to the Sporting goal and were rewarded by a somewhat dubious penalty award on 72 minutes when David Cassidy went down under a challenge from Stephen Paisley that looked outside the box.
McAllister shot the penalty under the body of Quigley who initially looked to have stopped it.
Anthony Flood was then farcically sent off for pushing Quigley over in trying to retrieve the ball, his second booking in five minutes.
Shelbourne: Delaney; Synnott, Brennan, Keely (Quigley, 56 mins), Hedderman; Baker, Cassidy (C O‘Brien, 85 mins), M O’Brien (Hynes, 56 mins), McAllister, Doyle; Flood.
Sporting Fingal: Quigley; Gannon, Maher, Paisley, Fitzgerald; James, McFaul, Williams; C Byrne, O’Neill (Corcoran, 90+3 mins), Zayed.
Referee: Tom Connolly (Dublin).