With the champions down to ten men following Lee Grace’s sending off, Bohemians drove home their advantage on 84 minutes.
Fellow substitute and captain Jordan Flores swept over a sublime cross from the left. With Leon Pohls inexplicably frozen on his line, Pole Piszczek arrived to head home.
It marked the most welcome of wins for Bohemians in every respect, their first in the league in 11 games.
Adding to their having knocked them out of the FAI Cup in July, Bohemians have now also surely ended any tenuous thoughts of Rovers retaining their title as they drop to sixth place in the table, 13 points behind leaders Shelbourne.
Indeed, ending a week in which they learned their opponents for their upcoming league phase of the Conference League, Rovers now face a battle to qualify for Europe next season.
In a frenetic first half of end-to-end action on the uneven surface, Bohemians Kacper Chorazka was the first goalkeeper to be worked when turning a Marc McNulty shot clumsily around a post.
And there was further worry for Bohemians in the ensuing minutes.
The well-positioned Leigh Kavanagh first cleared off the line from McNulty following Graham Burke’s long throw before Danny Grant tracked back to get a block to McNulty’s shot after he had worked a neat one-two with Neil Farrugia.
But there was no reprieve for the overworked Bohs rearguard as Rovers were deservedly in front on 15 minutes.
Referee Paul McLaughlin played an advantage after Darragh Burns was fouled by Paddy Kirk despite getting his pass away to Farrugia.
The winger found himself in a pocket of space to surge forward and strike a low drive that bobbled several times before finding the bottom corner of the net.
Rovers maintained the pressure with McNulty having appeals for a penalty not entertained when he looked to have been pulled back by Dawson Devoy.
It was the 21st minute before Bohemians seriously threatened, Ross Tierney shooting into the side netting after good work by Grant and Rooney.
Three minutes later a defensive mistake brought their equaliser.
Grace got little purchase into his clearing header from a Devoy pass down the inside-right channel.
The ball drop for the lurking Rooney who cleverly set up Tierney to sweep home from eight yards.
Either side might have scored again before the interval.
Rovers’ Burns was first guilty of a dreadful miss when skying over the crossbar after Chorazka had parried a Burke drive.
The frame of Pohls’ goal then rescued Rovers, the lively Rooney’s shot coming back of a post after skipper Adam McDonnell and James Clarke did well to quarry the opening.
A slow burner of a second half meant it was the 64th minute before either side got sight of goal, Rooney shooting over Pohls crossbar after the Hoops’ keeper cleared the ball straight to hime.
The game then swung Bohemians way on 66 minutes when Grace saw a second yellow card for a lunging tackle on Piszczek and was sent off.
Piszczek then delivered his big moment before Chorazka made a telling save in stoppage time to thwart Johnny Kenny.
Bohemians: Chorazka; Smith, Byrne, Kavanagh, Kirk (Flores, 81); McDonnell (Meekison, 81), Devoy; Grant (McDaid, 81), Tierney, Rooney (Greive, 90); Clarke (Piszczek, 60).
Shamrock Rovers: Pohls; Hoare, Lopes, Grace, Honohan; O’Neill, Watts (McEneff, 79); Burns (Cleary, 79), Burke (Mandroiu, 62), Farrugia; McNulty (Kenny, 62).
Referee: Paul McLaughlin (Monaghan).
