Given that both sides have seen their league title ambitions dampened due to Dundalk’s impressive start to the campaign, the importance of the tie was not lost on either side in a frantic opening.
Within two minutes, Michael Drennan was released through on goal from Jason McGuinness’s misplaced pass but a lack of composure culminated in a rushed effort that veered well wide of Brendan Clarke’s left-hand upright.
Ryan Brennan was next to spurn a presentable opportunity as he headed over from six yards following Keith Fahey’s precise centre.
At the opposite end, Pats were struggling to make an impression but from a Killian Brennan corner-kick, Sean Hoare saw his flicked attempt blocked on the line.
The landscape of the tie altered dramatically in the space of five horrific minutes for Rovers as they conceded twice and they were architects of their own downfall.
In the 32nd minute, a sloppy pass from Patrick Cregg was seized upon by Conan Byrne and the hosts’ top scorer capitalised superbly with a curling effort low past Craig Hyland from 20 yards.
Within five minutes, the lead was doubled as McGuinness rose highest to head home Byrne’s corner-kick, albeit after some poor handling by Hyland at his near post.
For a team not used to conceding, the double blow left Rovers chasing the game upon the resumption with manager Pat Fenlon introducing both Stephen McPhail and Kieran Marty Waters into the fray.
The crossbar denied Rovers a route back into contention within two minutes of the restart as Drennan’s close-range header cannoned to safety after Simon Madden’s pin-point cross.
Given their more adventurous approach, the Hoops became susceptible to the counter-attack and Aaron Greene should have done better than prodding an unconvincing shot against Hyland when released through on goal in the 48th minute.
Waters was proving a nuisance for the home defence and a fine pass placed Drennan clear in the inside-left channel but his resultant shot nestled in the side-netting.
Pats were indebted to Hoare who blocked a goal-bound effort from substitute Gary McCabe in the 75th minute.
Greene should have settled the issue four minutes later but for Hyland’s brave block.
The visitors piled on the pressure for the last 10 minutes but the Saints withstood the onslaught to confirm the precious win.
Clarke; Desmond, Hoare, McGuinness, Bermingham; Byrne (Langley, 78), Chambers, Brennan, Greene, Forrester (McCormack ,81); Fagan (McGrath, 54).
Hyland; Madden, Kenna, Blanchard, Byrne; G Brennan, Cregg, Fahey (McCabe, 65), R Brennan (McPhail, 46), Miele (Waters, 46); Drennan.
Paul McLoughlin (Monaghan)