A Mikey Drennan brace and a late third goal from substitute Brandon Miele secured the points for Shamrock Rovers at Tallaght Stadium.
Former Aston Villa youngster Drennan showed his predatory instincts, clinically firing past Conor Winn twice and Rovers sub Miele put the icing on the cake when his clever one-two pilfered a third goal for the Hoops in injury time.
Tommy Dunne had a long injury and suspension list to contend with heading into the game at Tallaght Stadium.
Centre-back partners Stephen Walsh and Sam Oji were both unavailable due to suspension.
David O’Leary was ruled out with a knee injury and Ryan Connolly took his place on the bench as he looks to recover from a hip injury.
Three changes were made to United’s starting eleven, returning from injury was Marc Ludden along with Kevin Garcia and captain Paul Sinnott.
A long ball out of the Hoops defence from Simon Madden caught United’s defence unaware and Mikey Drennan bore down on goal, United keeper’ Conor Winn rushed from his goal and spread himself to deny Drennan a dream start.
The same routine was repeated by Rovers in the 18th minute but this time to better effect. Madden launched the ball long, Gavin Brennan peeled away behind the United defence, Conor Winn rushed from his goal and the former Drogheda winger poked the ball past Winn to Mikey Drennan, who rolled the ball into the open net.
Madden, who operated as a second winger at times, rampaged down the right flank in 28th minute before crossing to Ryan Brennan at the near post but he could only hook the ball into the side netting.
Just after the half hour mark, United had their best spell of the game. Alex Byrne split the Hoops defence with a brilliant through ball to find Jake Keegan but Craig Hyland saved the American’s left footed effort.
Hyland was called into action again a minute later when Jason Molloy angled a shot at the bottom corner but he found the Hoop’s netminder in flying form and Hyland pushed the ball out for a corner kick at full stretch.
Andy O’Connell almost levelled matters when a Galway free kick caused panic in the Rovers box but his goalbound effort was hooked away to safety by a Shamrock Rovers defender.
The homeside ended the half purposefully when winger Kieran “Marty” Waters drew an excellent save from Conor Winn and a moment later, Mikey Drennan blasted over the crossbar.
Early in the second half, Keith Fahey broke into the United box but he was forced wide of goal and he dragged his shot inches wide of the far post.
Conor Winn had to be alert just before the hour mark when Keith Fahey swung in a dangerous free kick, Ryan Brennan won the aerial dual but Winn saved well.
The home side upped the intensity in search of a second goal and Marty Waters saw his long-range effort pushed out for a corner by Conor Winn.
A second goal did arrive for Shamrock Rovers in 68th minute when David O’Connor squared the ball for Mikey Drennan, whose shot deflected off United defender Andy O’Connell and wrong footed Conor Winn.
The returning Marc Ludden was given his marching orders by Rob Rogers in the 77th minute after he received his second yellow card of the match for a tired looking challenge.
Shamrock Rovers substitute Brandon Miele put some gloss on the home sides win in the 91st minute when he played a neat one-two with Fahey before blasting past Conor Winn into the roof of the net.
SHAMROCK ROVERS (4-2-3-1): Hyland; Madden, Webster, Kenna, O’Connor; Cregg, R. Brennan (McPhail 71); Waters (O’Connor 71), Fahey, G. Brennan (Miele 84); Drennan.
UNUSED SUBSTITUTES: Murphy (GK), Blanchard, McCabe, McCaffrey.
SCORERS: Drennan 18, 68. Miele 91.
GALWAY UNITED (4-4-1-1): Winn; Horgan, O’Connell, Garcia, Ludden; Molloy (Connolly 57), Sinnott (Raftery 92), Byrne, Shanahan; Curran (Cunningham 57); Keegan.
UNUSED SUBSTITUTES: Gleeson (GK), Barry, O’Laoi.
BOOKED: Ludden 31, 76. Shanahan 73, Cunningham 80.
SENT OFF: Ludden 76.
OFFICIAL ATTENDANCE: 3,298.
REFEREE: Rob Rogers.